The holiday takes a different tone here than it does back in the US, apparently. I grew up in a very.. working class? (no idea how to describe it here) part of Washington DC. It was, as it is in most parts of the US, a holiday which is mostly for small children. We'd get the odd teenager or two in that neighborhood, but that's mostly because hey, free candy. When we moved out to the more middle class suburbs, teenagers were a lot less likely to attempt to trick or treat because it was considered babyish.
What seems to have happened here is youths are taking the 'trick' part very seriously. :D We don't. Tricks happen, but usually only to people you know and dislike, or know and do like, but you want to mess with them. I think children learn the process by going with their parents from an early age, and for the most part thank people when they get candy, etc.
I'm not going anywhere with this comment, but I guess I'm saying, people back home respected the free candy. [hgrin] I grew up in a city that was known as the murder capital of the US and I swear to you, I never felt the sort of fear some of you are describing here. Some of them might have acted tough but you could just mess with them a bit, say you're waiting for a smile or something first, whatever.
It's a different holiday back home. And yes, I'm also 
by the 'nasty American import' thing, but everyone's entitled to their opinion.