I have to correct you - it was a RAP concert, not ROCK concert. They're not the same thing, even if both are a cacophony of instruments pretending to be music.
That said, the shooting occurred OUTSIDE the venue. No issue about the venue preventing people with guns inside.
And with a 'debris field' by the entrance, including 'empty tequila and cognac bottles', I suspect there was alcohol and/or other controlled substances involved. I'd also bet money that the shooters were not the legal owners of the guns used, either.
Damn thugs.

What is the point of rules that are not enforced?