I have not heard this! I don't really live IN the city, but in the burbs and my burb is pretty small, though we're surrounded by a lot of other bigger burbs. ANYWAY! I wouldn't doubt if it happens in certain places but I don't know any better because I don't have kids and don't know many with kids LOL.

My brother took his kids down to the city to trick or treat the Saturday before. Apparently they had some fun event in one of the plazas. My BFF takes her kids by her grandma's (also out in the burbs) where they trick or treat at the safety town, which is kinda reminiscent of what you're talking about but a few less car trunks involved :-P