It's about the defaults. The problem with political debates is that it is difficult to contain them -- they are likely to grow (because for almost any topic you can find a political point of view), and they attract new people who are interested more in promoting a political idea than about improving their own rationality.

So we can either explicitly support the norm "we don't debate politics (unless there is an exception)", or we can either explicitly or implicitly have the norm "political debates are okay". We have the former.

Maybe there are other possible solutions, such as trying to contain politics in specific threads, this was tried in the past (if I remember correctly, some people kept making more and more treads for debating NR pretending to be general political threads; or maybe it was other way round and all general political threads were hijacked to debate NR).

In theory, it should be possible to debate politics rationally, but in practice, we have problems keeping the debates civilized.