First off, the swear filter isn't from PD, they use the same chat filter that the PS3/PSN uses for regular chat sessions on the XMB.
Second of all, you have to stop assuming that certain words are filtered. The truth is, most of these words are filtered not because of the words themselves, but because they have a combination of letters in them that consolidate another smaller word.
A couple examples:
Tsukuba. The word "Tsukuba" isn't filtered, the word "suk" is. Try it out, type out "suk" and it'll be filtered.
Bacon. The same deal. "Con" is filtered.
The PS3's chat filter apparently filters out many swear words in many different languages. From what I read above, "con" is apparently a french swear word and I've come across my fair share of english words that contain swear words in spanish and they're filtered out too, despite the fact that the word I'm typing means something completely different.
See the problem?