The filtering is hard-coded into the 'ware that runs the place. A lot of forum-wares have similar filters built into them, and (trust me on this...been there, done that) it's easier to deal with it as opposed to dealing with (for example) irate parents who complain that their children were exposed to inappropriate language in a forum post. 'Private' in private message simply means that your post will not be publicly available for viewing.
It may also be worth remembering that most forums are NOT a public democracy, but rather a privately-owned/maintained operation, and the owners can do as they like. You agreed to these terms when you signed up here. The crew here actually allows a rather large freedom to us in general, and I appreciate that. If you disagree with the management here, you can always delete your account.