For starters, when most forums ban people it's because they want them to leave. Allowing trolls and assholes free rein as long they're rich kind of runs counter to the whole point.
Having to get out your credit card to swear at people on the internet seems to do a good job of getting people to stop and reconsider. Not always, but it helps.
The SA account cost and moderation work very well and do not result in a forum full of rich trolls. I've seen other forums use similar tactics (small cost of entry and strong moderation) to preserve a high signal to noise ratio.
The trolls and assholes are still cleaned up after. There are people who spend a few hundred bucks trolling but they're essentially invisible unless you go looking for them.
