That's probably because of the groups of youths who go around that are unpleasant if they don't get anything when they knock, or throw eggs and flour etc no matter what. The attitude of those who 'trick or treat' in the UK is probably not the same as that in the US.
I also don't get Halloween from another perspective. For the rest of the year children are told not to speak to strangers or to take sweets from them, yet on Halloween it's OK for them to knock on strangers' doors and accept sweets from them. Talk about giving kids mixed messages.