It is not ridiculous, since people aren't downmodding to express a difference of opinion about what the person said. They are downmodding to assert they do not want to hear any political opinion on Hacker News, because--as has been said--there are countless other venues for hackers to express and debate political opinion; Hacker News is not one of them.

Notice therefore we see something interesting regarding the HN system. In essence, this is exactly what pg had in mind. Karma is taken away for comments and not just submissions, so that users will adapt to the Hacker News atmosphere. As we have seen from this thread, politics (even "news" like the first ever black general election presidential candidate) is simply not something people are interested in seeing on HN, and if people keep discussing it, they will be downmodded into oblivion, effectively maintaining the current HN mindset (since, presumably, people with negative, or quickly decreasing karma, will not want to continue posting on HN).

The only thing that could break such a system is a vast shift in the mentality of the community (which is not a bad thing since it would still keep the community happy, although it's very unlikely to happen), or a massive influx of new people into the community (which is why pg wants to keep the number of new signups per day low).