BLM and its concerns for justice is considered a very political topic. Black issues in America aren’t about to become an unpolitical issue.
An oft repeated argument is that political activist speech kills curiosity. There’s no transparent line to know when political speech is curious enough not to violate site rules.
Also, where does the mandate of the site come from? The president of YC? My worry is that the prescriptions of the site don’t seem to have any story for evolution.
https://news.ycombinator.com/item?id=23540162
https://news.ycombinator.com/item?id=23564048
https://news.ycombinator.com/item?id=23772359
There's plenty of curious conversation there. There's also plenty of flamebait and flamewar, unfortunately, but that's unavoidable when the society at large is divided on a topic—or rather societies, since we have the added dimension of being a highly international community to deal with. The HN guidelines are written in such a way as to encourage the former and discourage the latter, but there are limits to what's achievable.
HN's mandate comes from how it was created. It has its particular niche. I think it's a good niche that is worth preserving, and I'm pretty sure the bulk of the community agrees, since that's why people come here. In a way, I like that you're questioning it, though. If the argument becomes "HN should have a different mandate", this suggests that it's doing an ok job of fulfilling the existing one.