Let's keep in mind that the amount of civility is roughly inversely proportional to the amount of controversy. Posters in other threads who are more civil aren't that way because they're nicer or calmer than posters in the politics thread. They're more civil because it's easy to be civil when the stakes aren't very high or important. Whether Contador or Froome has had a better career, or whether one musician is more appealing than another, are questions that really aren't that earth-shakingly important to fans of those topics. What happens to someone's country matters a whole hell of a lot more, particularly to those who believe they have little power to change the system.
I'm sure anyone who has ever complained about the politics thread gets very angry and incivil over certain issues, it's just that when that happens, it's rationalized as justifiable. If you see a parent abusing a child, e.g., you probably won't approach that parent and politely suggest that he or she stop treating the kid that way. You will likely get very angry, and you will believe with all your heart that you should be angry--that only by being angry can you emphasize how deeply you believe that that behavior is wrong, not something up for debate. Much incivil behavior in political discussions is like that; social issues can become very personal and immediate. It's just that those not passionate about those issues don't recognize that in other circumstances they would behave pretty much the same way.
That was certainly the rationale in the Daily Peloton forum. But it can work the other way, too, as political rivals often found common cause vs. LA, e.g.