I live in this kind of neighborhood. We all help eachother out and watch for eachother's kids. One of the reasons I wouldn't move, actually. I never once perceived it as picking up slack for another mom. We all do this for eachother. The kids play at different houses and the mom who is there steps in if needed. I think this is pretty typical, actually.

I think your examples are rather extreme and pretty rare (at least where I live). Kids aren't throwing rocks at cars, etc. In my neighborhood, when I have a group of kids on my lawn playing (quite often, actually) I'm ordering pizza, throwing hot dogs on the grill, bringing out ice pops. I don't see it as a job to watch these kids. I'm glad my kids have so many friends and that the neighbors love coming here to play. I watch out for their kids when they are here, as do they when my kids run down there. There's always a group of kids hanging out together going to play randomly on about 4 different lawns, all of which I can see. That's how our neighborhood is, and most of the moms I know, we all watch out for everyone's children.