actually, arguing that fraud filters must be private is interesting. Would you make the same argument about keeping encryption algorithms private? :) The two concepts are similar but not exactly the same. Unlike encryption algorithms, a fraud filter is not bullet proof. It is just an alert mechanism. So, yeah, it probably needs to be kept secret.
Encryption algorithms are fraud filters are very very different (as you already mentioned). Most functions used are one way and the inverse is (to the best of our current knowledge) intractable (or to be more technical - their complexity class is unknown and widely thought to be intractable). So, it can be made public. Requesting fraud filters to be made public is like asking Google to make their spam detection public. This can (and probably will) defeat the purpose.
