If you don't understand how to conduct yourself on a forum where your interests are not the only interests, then there's a chance you'll get banned. When your personal point of view conflicts with the interests of those providing and moderating the forum, then there's a greater chance of getting banned when speaking out.
The chance increases depending on how hard it is for you to respect the interests of other people.
It's pretty simple, really.
I rarely post on official forums, because I don't really see the point. But I often read them - especially when trying to solve technical issues or to get some idea of the future of a product.
I find them most useful in that way, actually.
My experience is that you sort of have to go out of your way to get locked out or banned. Well, that, or you're just not very good with people - and it will just come naturally.
I've had a couple of warnings over the years - since I tend to care very little about whether I get banned or not, and I don't enjoy communicating if I feel I can't be honest about it. But I don't think I've ever been banned.
I've banned myself from a couple of places, though - like the QT3 forums and a few other places. Sometimes, I recognise that my opinions are not welcome at all and that people don't seem to enjoy my contribution - so I just leave. With QT3, I quickly discovered that I didn't fit in with the people in "power" - and I don't know why I even registered there.
I guess I just enjoy exchanging no matter what