Political topics attract participants inclined to use the norms of mainstream political debate, risking a tipping point to lower quality discussion
(I hope that is the least click-baity title ever.) Political topics elicit lower quality participation, holding the set of participants fixed. This is the thesis of "politics is the mind-killer". Here's a separate effect: Political topics attract mind-killed participants. This can happen even when the initial participants are not mind-killed...
Good points. This may be another case where we evolved to have probability-weighted-by-utility intuitions, and where we work backwards from these intuitions when ask for a model of raw probability.