The proxy angle is interesting, but I don't think that's main factor here. What's walking through someone's lawn a proxy for? The cops see a kid who looks like trouble and who's technically breaking the law and they arrest him, whether the specific crime is a proxy for anything or not.
And then there's the question of what "the spirit of the law" means. The courts wouldn't allow a blanket prohibition on "gang activity," even if that may be what legislature had in mind.
And then there's the question of what "the spirit of the law" means. The courts wouldn't allow a blanket prohibition on "gang activity," even if that may be what legislature had in mind.