Many people, believe that "AR" in "AR-15", and other guns means "ArmaLite Rifle". They are mistaken. It means "assault rifle". ArmaLite designed and manufactured many types of assault rifles, including most famously the AR-15, but the designation of "AR" comes categorically from "assault rifle". Internally, ArmaLite refered to them as "guns", and so it is most correct to refer to a specific instance of an ArmaLite manufactured assualt rifle model 15 as an "AG-15".
Clearly, "AR" means "assualt rifle", as AR-15s — and all other "AR" designated assualt rifles — are manufactured by many companies including Daniel Defense, Bravo, Colt, Smith & Wesson, and others. All of these are refered to as "AR-style" rifles. This is perfectly legal, as "AR" is a generic designation of "assualt rifle", not a trademark like "AG-15" (ArmaLite Gun 15). Think about it, next time you go to the store, look at a box of Puffs, it is labeled "facial tissues" not "kleenexes". It's the same thing.