I think most people would accept that thoughtful, evidence-based discussion should be discouraged while sitting in the front row at an opera, or in the midst of a professor's lecture.
What I'm describing is similarly an issue of logistics, not content. I'm not making a claim about sensitive topics, and I'm certainly not proposing dishonesty or the suppression of uncomfortable truths. The problem with off-topic content is simply that: it's off topic, and on a forum thread or the top of an HN post it makes on-topic conversation more difficult to conduct. Forked-discussion settings like Tumblr and Twitter are closer to a conference than a lecture, and can sustain popular off-topic discussion with less derailment.
The relevance of politics and sensitive topics is only in my second point, that places like HN which center on non-political topics can create particularly good discussions. I largely agree with you, I'm endorsing the fact that HN doesn't ban politics or sensitive topics; the rules of avoiding flamebait, grandstanding, and excessive derailment help to prevent pointless yelling while preserving good political discussion around the margins.
(As an aside which risks being off-topic: why David Geffen? I've never seen someone use him as their go-to example of sucking up to a billionaire.)