The 1994 assault weapons ban identified so-called assault-weapons by name and by evil features. 2 or more evil features makes a semi-automatic rifle an assault-weapon, IIRC. The evil features include a flash suppressor, pistol grip, folding or collapsible stock, bayonet lug.
The AR15 has a pistol grip, that is its one evil feature, so it can't have any more evil features like a flash suppressor. The M1A doesn't have a pistol grip, so it can have a flash suppressor and still have only one evil feature.