'may not' is only slightly ambiguous in general ("is not allowed" is preferred to "can choose not to"). In this context though, it definitely takes on the "is not allowed" meaning because thee negative, "they may assault" was not used.

That is though the literal meaning is ambiguous, the implication (because of not using the negation) forces the interpretation to be "is not allowed".

'may not' is only slightly ambiguous in general ("is not allowed" is preferred to "can choose not to"). In this context though, it definitely takes on the "is not allowed" meaning because the negative, "they may assault" was not used.

That is, though the literal meaning is ambiguous, the implication (because of not using the negation) forces the interpretation to be "is not allowed".