Not a downvoter (and I doubt this is why people are downvoting, but I suppose it could be) but I somewhat disagree. There is a (very small) group of friends with whom I can discuss political topics without them becoming mind-killing. Frequently this is due to admitted information gaps between us, and it's about learning the specifics of an issue. It's certainly not easy though, and often involves us switching sides on each other when we see one of us using dark arts. Similarly, I would expect most LWers to be able to (somewhat) rationally discuss politics. Which is not to say that they would enjoy it/should just that I would expect them to be far more able (moreso than my friends) to do it productively.

If I had to guess why people are downvoting, though, it would be because discussing politics can bring you more happiness, and indeed does for many people, even if it's just yelling at each other. Although I feel very uncertain (p = 0.35) that this is actually the reason for downvotes.