Because you're playing it as a conservative, and arguing with them, I'll bet.
It's possible that the direct approach might work. If someone were to log in and say, "I'm a Freeper, but I'm not here to argue. I just want to see why Freepers and DUers think so differently. A number of us took this Jungian personality test (link), and it would be interesting to see how DUers score on the tests." It's possible that it would pique their interest enough to play along.
A more likely approach would be to establish some cred, by slinging LIHOP and MIHOP and BFEE around for a few weeks--that could be done with a shell script--and then starting a thread saying, "Hey, look at these idiot Freepers! On this thread (link) they all outed themselves as INTJs!!!1 What a bunch of saps. I'm an ESFP, how about the rest of us?"