Mainly because most demo users come on the forums, start bitching about the game, ask for more cars, tweak, tracks, blahh. Ask with poor grammar, and constantly nag. TBH, theres few demo users here I can actually have a heated discussion with. Not to mention the legal issues we run into with some demo users...
But yes, as Android said, as long as you are respectful, post maturely, and are kind you won't ever have a problem. Only problem I ran into was people catching on about my S2 license (which is valid, very few people keep their demo forum account while using S2)
~Bryan~