Chris and Yeahha basically summed it up. Banning someone isn't a single decision, especially permanent bans. It is the result of repeat offences where the person just doesn't want to follow the site rules that he or she agreed to follow. We ban a lot of users per day (vast majority are spammers) and there are times when we have a report that requires more attention, especially if its someone with a track record. Several of us put forth our opinions on the matter and come to a conclusion on what we believe should be done. Some of us are a bit more lenient than others but the end conclusion is from a collective agreement. Banning someone is a matter that we don't take lightly and that we wish we really never have to make but like society, not everyone wants to follow the agreed upon rules.
Post count really has nothing to do with bans since we never take that into account. Banning someone with 50 posts who makes posts attacking other members or hate speech would be exactly the same as banning someone with 5000 posts. Warnings, probations, temp bans, longer temp bans, and permanent bans is the typical route that is taken... So for someone to be permanently banned, that either takes work over the time that he or she was a member or a few select posts where they spout hate rhetoric (as one example).
One easy way to not risking yourself of getting in trouble... Avoid the Politics and Current Events subforum. Thatll decrease your chances by around 73.5% +/- 20%.