Because that is where most of the software is developed - or it is developed primarily for the American market.
They can alter the swear filter to only include 'whole swearwords' stopping censorship for proper words, like the S c u n t h o r p e example. But that will mean more vigilant monitoring by the moderators to search out instances of abuse - or better reporting by the members.
Until forum software starts getting 'intelligent' with grammar checking etc. everything has to be done by humans. That is quite a way off - as most servers don't even support spell checking yet!