A little over a week ago, my gaming group (with a friend dm'ing) actually got around to asking for the logic of Calistria having the knowledge domain. It took me a moment to think about it, since I'd never considered it, even though I'm playing the party cleric (a Calistrian, of course, and started back in pathfinder beta), before I could come up with an answer, and I'm sure that others could expound on the idea too.
My rationale for Calistria offering the Knowledge Domain is rather subtle, and actually does not necessarily feed through to the actual abilities of the knowledge domain completely. As she is the goddess of lust and revenge, I stopped to think about what specific uses she would have for a lot of these spells/powers, and a lot of them are not necessarily focused on adventuring clerics in this case. First off, the knowledge domain 1st level power is one that probably really doesn't see much use in combat, since the cleric usually doesn't want to get close enough to merely touch an opponent in combat. However, out of combat, the touch with its knowledge check are actually very applicable to someone focused on lust, as the knowledge gained therein could, by dm ruling, be themed to find out about sexual preferences and other sexually focused knowledge. As for revenge? That's another one where one of the abilities works really well toward her interests. Remote viewing, which you gain fairly early, allows you to peer through walls/locked doors to see things that could theoretically be used for blackmail, which offers some very interesting role playing for revenge. Anyone else have specific religious reasons why she would offer that domain?
Her clergy are specifically called out as being information brokers, and leveraging secrets about people for gain (personal, or otherwise).
Yep. You can learn all sorts of things from pillow talk, and the whole "sacred whore" thing that Calistrian temples go for is perfect for gathering and distributing illicit information.