1. There appear to be some inhibitions about certain political topics. Daily Kos started a series about taboo topics in 2004. (Introduction, Part 1, ...?)
2. There are certainly some areas where you can be pretty sure of a strong (negative) reaction, possibly from people who haven't paid close attention to what you've actually said. If those who generally talk about these subjects are idiots and/or bigots, then you can understand why people are often unwilling to invest time and energy picking out the needle of common sense from the haystack of rubbish.
3. There is also a possible inhibition against political discourse as a whole. In his post The Taboo against Political Discourse (no longer available) Masood Mortazavi quotes Lawrence Lessig:
"It's fine to talk about politics with people you agree with. But it is rude to argue about politics with people you disagree with. Political discourse becomes isolated, and isolated discourse becomes extreme. We say what our friends want to hear, and hear very little beyond what our friends say."
What is the purpose of political discourse? To change minds and win hearts? When I was young and idealistic, I used to think that there was some purpose in arguing with people, because there was a reasonable chance of changing their opinions and beliefs. I gradually realized that this didn't happen very often, and that people mostly argued in order to affirm their own opinions and beliefs.
So if people cannot be bothered to argue, is this a taboo or merely political ennui?