Because almost with 100% accuracy, any email with the word "perl" in it is not spam. And any email with certain spammy words in them, are spam.

Think about the kind of marketing crap a CEO might subscribe to though. That's the sorts of things we have to deal with in SpamAssassin.

However, now the cat seems to be out of the bag with bayesian filtering, we may as well provide some code to let people who run it themselves do their own customisation and allow it to run for them.