I'm sure doing all that becomes a coding issue that would take too much time. But as far as seeing the bad language Andrew decided to have his site of a PG/PG-13 rating at the very very most. It's just an excersize in restraint to watch the language and it's very easy to say junk, dang, or darn in the place of vulgarity. Because once again, it becomes a coding issue that would take up too much time that would take away from other aspects of the site.
But even if PBN 'allows' it to a certain extent when you sign up I do recall seeing something that said 'if you select this feature this does not exempt you from the other restrictions of the site and you can be subject to a ban as per you language choices'. So even they can ban you for language if it gets to excessive but Andrew and the other mods like to take a pre-emptive stance to try and stop the problem before it starts.