For "**A is B**" the explanation is simple.

Gensler's language has two types of "basic" formulae:

(i) formulae expressing relation between sets ("general categoris"): "All logicians are charming", translated as "**all L is C**"

(ii) formulae expressing the fact that an *individual* belongs to a set: "Gensler is a logician", translated as "**g is L**".

In this second case, **g** is the name of an *individual*; thus, we cannot quantify it with "all" or "some".

In the previous case, instead, **L** and **C** are names for sets and we have to quantify the first one in order to correctly express the relationship between them. If we say "Logicians are charming" (i.e. **L is C**) we have an ambiguous expression, because we do not know if we are asserting it of all or some Logicians.

I presume that he forbid the expression "**all A is not B**" as "ungrammatical" (*non-wff*) simply because it is already expressible as: "**no A is B**".