The way I see it, I don't think these topics are relevant to the HN audience. I know that they are probably de facto HN topics because so many readers live in the US, but HN is supposed to be about tech regardless of where you live. For that reason I would always flag an article about weapon laws in the US, and I would expect most of HN to think the same (except maybe on weekends, when things are a bit more relaxed).
I see where you come from, though: the discussion in HN is for the most part polite and informed, and it would be nice to have more of that in general. But I fear that if you started adding "topics" to HN you would dilute it and lose what makes it special.
"On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity."
But I put it to you that some topics -- such as US politics -- are less likely to gratify one's intellectual curiosity than to provoke reaction and negative emotion.