Why? The 2nd amendment gives us the right to keep and bear arms, but most of us realise that the right is not unlimited, for example most people don't fight the ban on fully automatic weapons (machine guns), and there isn't a lot of people calling for the right to own grenade launchers or RPGs, even though they are "arms" as well.
So really, a vast majority is in AGREEMENT that there is a line between "acceptable" arms and "unacceptable" arms.
The only question is where you draw that line. We can argue over that line, we can say people are stupid for where they draw a that line, but it's hardly an oxymoron to suggest that the line exists. We all know it does.
To attempt to relate the AWB to machine guns and grenades is liberal spin that would make Schumer proud.