They're not, and they can absolutely decide to host or ban whatever they want. But I don't think they should.

When you start banning things you don't like, you end up creating an echo chamber, and then you eventually get to the divisiveness that we have today. Liberals flock to X, Y, Z platforms, and conservatives flock to A, B, C platforms. That's a problem, because it eliminates any kind of cross-pollination of ideas.

That said, not all ideas are worth hosting. Harassment is never okay, so any individual, community, or instance that protects those who harass others should be blocked. But just having different ideas shouldn't be grounds for blocking.

We should absolutely not support cancel culture in any form, we should instead encourage dialogue. Instead of blocking people that think trans people are gross or whatever, we should be open to explaining how those views hurt real people. If you convince just one person, they'll influence others and we'll make progress toward broader acceptance. I draw the line at actual harassment, but ignorance shouldn't result in a ban.

So that's why I urge restraint when blocking communities. We really don't want to go down the road of blocking things we don't like because that just puts up walls that fragments an already fragile community.