You guys know that helmets don't protect from bullets? Army helmets are for protection from shrapnel and dirt thats flying around. Do the drug lords fight with mortars and artillery grenades? The normal helmets also make hearing difficult which is bad in urban combat (hence you see more and more helmets with ears open in places like Iraq). So i'm not 100% convinced that their no helmet appearence has to do with bad equipment. Could be a tactical choice.
And it also makes sensde that some of these guys have FAL's. Not all of them fight inside buildings. Afaik they have fights up hills and alleys with longer combat distances. It's a good weapon to actualy stop a criminal on mid-range.