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.