In a democracy, I don't think this is an appeal to authority anymore, nor is it entirely circular.
It is clear that laws are based upon the shared moral sentiments of the population, if we are electing our legislators and even our judges. They have authority, but it is our authority.
We may not feel represented by the composite, here, in the same way that the current form of the English language may not be the language we would choose to have, and we might disown, disparage, or ignore parts. But despite the unequal representation, various power relations and affordances to our own weaknesses and external pressures that any social compositing process involves, the rules our society in general enforces are our composite moral sentiments. We control them and they are built out of our coordinated decisions. We change them by winning or losing arguments about what is right, and they have no other source material, absent some outside antidemocratic force. (Capitalism does not really qualify, it is hardly 'outside').
The dark forces that shape our politics and form the worst aspects of our laws are just parts of our morality that we would rather not discuss. The analogy with language as a similar form of social construct holds: For instance, we have a language that chooses given forms of sex which are presumed to be unwanted as the primary metaphor for advantage-taking, lack of consideration, and bad luck. And that does represent a dominant culturally shared opinion of the people who take the roles alluded to in those metaphors. I may find sucking and being screwed to be glorious opportunities worth seeking after, and so might a whole lot of women, but we in the composite, as a culture do not wholly approve. We can then claim the culture really agrees with us, because we are on the more 'speakable' side of the issue right now. But when we do, we know we are lying. Otherwise, our shared aversion would change the language over time.
Then isn't this sentiment just the fallacy of affirming the consequent? Laws represent our shared sense of order, including large parts of our composite morality. So to take them as determining morality is following the implication of the definitions involved in the wrong direction.
However, one can follow any induction backward correctly in the negative, and there is neither a philosophical position, nor a fallacy involved. So it really depends upon whether you are arguing a necessary or a sufficient position, and where the negations of principles fall.
You cannot determine that an act is moral from the law, but you can deduce that many are morally compromised. You can deduce, for instance, that unexpectedly violating the laws protecting other people, based on your own personal decisions, is at least partly immoral. People have entered into the institution of citizenship (voluntarily or otherwise) primarily for the purpose of stability, and you are depriving them of that stability. Unless you forfeit all the associated privileges, you are acting destructively and in bad faith. (Exceptions apply, but the argument, as far as it goes without falling prey to other issues, has some real moral force.)
Appeals to authority and other circular arguments do not have this feature of having real applicability in one direction, but not in the other.