You haven't given the matter sufficient thought.
Odds are these are gangbangers. Guess what? Gangbangers aren't "checked out" and they don't buy their guns where any authorities have any say so about whether it's "allowed".
If a bunch of guys are willing to have a shootout in the middle of a mall (agg. assault and possibly murder) do you think they care about a penny-ante gun law?
The only thing gun laws do is disarm the law-abiding, and reassure the criminals that nobody will oppose their depredations.
It may be hard for you to understand, but that's because you're not thinking, you're emoting. Emotion feels good, I guess, but it doesn't solve problems.
btt
Shades of Demolition Man. Life immitating Hollyweird "Art?"