The problem is these people reproduce like cockroaches, and they_don't_watch_their_kids. They pretty much all move into the same neighborhoods, so as a result you have these huge parts of the city full of feral children.
I live in a suburb of L.A., and the gang problem out here is fucking unreal. My dept. just moved to a new office in a bad part of town 3 months ago, and so far 6 building windows have been shot out, 1 car tagged w/spraypaint, 1 car's gas siphoned and 1 guy's truck broken into and his ignition pried loose.
There are a hell of a lot of mexicans out here, and the ones that got here legally tend to be the ones that contribute to society. The ones that didn't are also almost always the ones that play their fucking music too loud and/or too late, pack 18 people into a 3 bedroom house with 6+ cars on the street and bring down people's property values. There are just too many of them.