I think the argument is that assault rifle has become a politically charged label that is meaningless. A rifle with a detachable magazine is just a rifle.

Generally I agree that using this sort of thing to get a 'gotcha' moment out of it is a waste of time. The problem is that context doesn't help me understand what a person means when they say assualt rifle. I've had someone describe an assault rifle as anything that's specifically made to kill people.