I was waiting for someone else to ask first.... Why DO they get banned?
I will give you an example, but so that we do not embarrass anyone it will be a hypothetical situation.
This is just one of a million scenarios that could happen.
A member posts something calling another member a name or saying something insulting to them. A mod or admin would send a PM to the person after removing the post telling them that when they said xyz that was insulting and in violation of our community policies (which we would sent them a link to as well). We would also tell the person that if they have any questions at all then they are free to discuss it with us via PM.
A few weeks later the same member starts a political discussion. We would again remove the post and let the member know that they violated the community policies (with link again). We would generally ask them at this point to make sure they have read all the rules and again let them know that if they have any questions then they can ask us.
A little while later the same member posts another insulting remark. We would again remove the post and again PM them member. This time it would be a stronger worded PM and we would most likey say that the next time they insult someone on the public board then they will be banned for a week. We would probably copy the part of the community policies that state that you are not allowed to insult members and if you have a problem with someone then you take it to PM, report the post, or alert a staff member by PM.
The same member insults someone again. We then ban them for a week and let them know why. We also tell them that if they break the rules again after their week long ban is over then they could be permanently banned.
Again, this is just one possible scenario. There are a million more. There are even a million different ways this very same scenario could go. This is just one way, off the top of my head, that it could happen.