Let me just preempt this by saying that I think you and tomhow do a very good job at moderating, and I'm just some goober on the internet sitting on a high-horse, so take what I say with as much respect as possible.
Hacker News is my favorite forum in no small part because this forum's users are, on average, a lot more educated than the average internet user. If not formally, a lot of the people here still do value learning and education as a whole. Those environments aren't organic on the internet, and it is largely due to efforts from folks like you to cultivate this audience and I do not want to dismiss that.
The concern, then, is that when the educated people can't discuss (and let's be honest, argue about) politics, then the only people who will be discussing politics will be the uneducated people. Politics is inherently contentious and we can't make progress (however you want to define it) without occasionally hurting feelings.
Now, a perfectly valid counter to this is "we're not stopping you from discussing contentious political issues, you're welcome to discuss it on one of the many other forums on the internet, just not here". That's fair enough, but it can come off as a little arbitrary, because virtually anything can be deemed "political"; I could argue that disagreements with type systems or the ISO standard of C or complaining about SQLite could be construed as "politically motivated".
I do realize that a line has to be drawn, though. The last thing I want is for the forum to devolve into 8chan or The Drudge Report or something, so while I don't completely agree at where you draw the line, I do understand why it is drawn.