I think they just figure that free speech is a good thing because it makes it easy to spot the idiots.The Washington Post comments section in general are a bunch of Left Wing Morons and Right Wing Morons yelling at each other. It is sort of surprising that a paper with the Post's stature would not exercise some control over it's forums.
Either that or they really don't care.
I particularly enjoy reading the comments for Charles Krauthammer's columns. At least half of them seem to think that Krauthammer personally reads their semi-literate screeds. Typical comment runs something like, "Kraut, you stupd jew NEOCON warmongger you dont have the Braines to understannd why the jews are hated by Evryone in the world why dont you get someone to PUSH you and you're wheelchair off a bridge you NeoCon jew."
BTW, the level of hatred hasn't noticeably decreased since January 20.