This kind of construction is rare in English, but it does occur. It is intended to be stronger than a simple command. If I were to try to explain why, I'd note that

Nobody touch him!

is a command. It is in the imperative, as you noticed. The implication is that you do not want anyone to touch him, and you will act against anyone who tries. This does indeed capture the intent.

We strengthen it with this phrasing

Nobody touches him.

This is now no longer phrased in the imperative. It is now phrased as a statement of fact. A peculiar construction for sure. Obviously it is possible for someone to touch him (all they have to do is reach out and poke him), so how can one make such a statement?

The best way I can describe this is that the speaker is declaring what their "world view" looks like. They are announcing that they are in a world in which nobody touches him. They're not even considering what they might do if someone tries to touch him, because in their world, nobody touches him.

Both phrases act as commands, but they are slightly different. "Nobody touch him," in the imperative, has a sense of "If you try to touch him, I will try to stop you." There's a sense that there are still some rules in place. If you're not the kind of person who would kill, people generally assume you wont kill them if they try to touch him.

"Nobody touches him" is different. By announcing the world view you are using, you are implying "If you try to touch him, I don't know what I will do." You haven't thought it through. People cannot rely on you to be yourself if they touch him. You just might snap and actually kill someone over this. You don't know, and they don't know.

The difference is subtle, and my fellow native English speakers may even disagree slightly on the shades of meaning. However, I find this explanation valuable to myself because it ties closely to the grammar of the phrase itself and works well with similar phrasings. And in the end, the answer to your question is "The movie is right, but you are right also!" The form you recommend, "Nobody move!" is the most typical way of phrasing that command. The form "Nobody touches him!" is a much less common phrasing, but it is still valid.

"Nobody touches him" also indicates that all the people hearing the order should prevent anyone else from touching him. "Nobody touch him" is just an order not to touch him.

The most common place this kind of conjugation is used is in a scenario like the following:

A robber bursts through the doors of a bank, grabs a young girl, holds a gun to her head, and says:

"Nobody moves, or the girl gets it!"

The key to "deciphering" the strange conjugation is that the robber's statement here is not a command. It's stronger than a command: it's a statement of fact. The robber here is saying "One of these two options will happen, and you cannot change this fact."

"Nobody [does action A], or [unfortunate consequence B]!" is the most common way for this type of conjugation to occur. But, in examples like yours, you may see the second half omitted. This is because either:

• the speaker wants to leave it up to the imagination of the listeners what the consequence might be
• the consequence is obvious to anyone looking (for example, the speaker has a gun, or the speaker's hand is hovering over a button marked "self-destruct")

Strictly speaking, you're not wrong.

But it's something of an idiom to take the form:

The rule I am now making is that nobody touches him. I expect you all to abide by this rule.

or:

I require that nobody touches him.

or:

From now on, either nobody touches him or I get cross.

and make it shorter.

Shrug.

(That second example may not actually be completely correct; I have a hunch that if we wanted to be *really* picky, one should say "I require that nobody touch him" but I can't remember and this is close enough to real-world usage.)

Interesting question. No one talks about the third person imperative mood much in English, but it is typically accomplished with a subjunctive conjugation:

Everyone be good.

Or, as you noted:

Nobody touch him.

However, it is quite common in colloquial speech to use a regular indicative conjugation. As long as it sounds like a command, it won't be misinterpreted.

There is a definite difference in these commands, depending of course on the context it was used.

"Nobody touch him!"

Is a direct command of warning given to those present. It is a warning that implies doing so will result in something bad happening. Perhaps pain to the person being touched, or contamination of the person doing the touching, or whatever.

There is no inherent accountability, "or else", to the first person in this command itself, though the way it is said can change that.

For example: Patient enters with an infectious disease, doctor orders "nobody touch him!". Anyone touching him after that is on their own responsibility. Should they do so, and get infected, response from Doctor would be "You are an idiot, I warned you!"

"Nobody touches him!"

Is a different kind of command. It tells those listening that you are putting them in charge of guarding the person and making sure they are not touched by anyone including them. It also implies that should someone touch the person, and they did not prevent it, that they will be held accountable for that. There is a definite, implied, "Or else" to this command.

For Example: Detective at crime scene tells cop regarding dead body "Nobody touches him!". Should Cop touch the body, or allow someone else to, contaminating the scene, Detective will have disciplinary action taken against the Cop!"

If the command is coming from a hoodlum, that punishment is usually lethal, at least in the Movies.

I thought "Nobody touches him!" is a partially-stated threat, not an imperative. It gives a concrete expected condition followed by an outcome which is unstated: "Nobody touches him, or I will do something terrible" / "Nobody touches him, or something terrible will happen". (In some cases the person making the statement has previously demonstrated or implied that they possess the means and temperament to do something terrible before this kind of statement, e.g. in a robbery/hostage situation.)

The outcome of the threat is left unspoken for effect; the unknown can be more frightening (or more dramatic) than the known as the audience automatically mentally invents some example for the unstated outcome, and something not stated is less easily refuted. (It is easier to rebut a specific threat than a vague one, although a general demand for more information can come into play: "Nobody touches him, or [else] what?".)

If the purpose of the statement is to strongly convey a desire on the speaker's part that nobody do the action, then the vague threat is not an actual threat, and its being unspecified allows it to bear rhetorical weight without bringing along specific details which would necessarily follow from a specific threat. (E.g. "Would you actually have stabbed one of us for touching him? We're your coworkers, and I thought friends!")

A simple, distilled version of the answer:

"Nobody touch him!" is an imperative which gives a negative command.

"Nobody touches him!" is expressing an intended rule, similar to "People without passports don't board the plane.". The rule can, of course, be violated, but the sentence communicates an intent that it be observed.

A very concise explanation:

• "Nobody touches him!" means "he shouldn't be touched - that's my order (in general)" whereupon "touch" is closer to"to harm someone, or use or damage something"
• "Nobody touch him!" means "don't touch him (at the moment)" whereupon "touch" is closer to "to put the fingers or hand lightly on or against something"

Both the sentences are imperative and both are commands. The former can be used when a person who issues the command doesn't want anyone to harm whoever isn't to be touched - in general. The latter can be used when whoever is not to be touched is close at the moment of speaking and probably infectious.