That's (in part) because the modification to bring it back into compliance with SAFE was a pitiful half-assed one. Basically he just needed a set of screw drivers and a vise to undo it. He didn't have to have smithing skills or equipment.
He fired fifty times to kill ten people. That's, well, that police level bad marksmanship. He would not have been able to kill that many if he didn't have the higher cap swapable mags. That's the thing about guns like this. A smart, determined, lucky, skilled, or clever sack of pus can kill a lot of people a lot of ways even without a firearm. These guns let moronic, lazy, unlucky, bumbling, dullards kill a lot of people reliably and quickly. Such people should not have access to them. If that means that almost no one should have access to them, well, that wasn't were I started last decade but it's becoming something even I could get behind at this point.