Nuances in their constitution dictate the legitimacy of such groups along with the interpretations of a right to be secure. Apparently it was in response to the colonial period where a peasant class was not allowed any right or means of security or protection for any purpose, they had to pay the next class up for their protection.

The Mexican version of RKBA translates to the people having the right to keep guns in their homes, but now "Arms" which the language of their constitution uses to differentiate military from civillian grade weapons, but interpretations are dictated by their equivalent of the BATF, which is run by their Army, which is in alliance with our BATF and by extension and web of intrigue, some but not all of the cartels.

The townspeople on the other hand are entirely within their rights to say that semiauto Ar15s or AKs, which are not precisely in military configuration are allowed under their constitution. Their government on the other hand, only wants to allow for non-military calibers and really basic Elmer Fudd profile long guns.