Facebook could be considered a public square, where they would be required to protect it's users free speech and lose their own ability to censor by choice. This would be similar to the central area of a mall or when companies provided housing to workers to prevent unionization efforts. Both places were upheld as places where the First Amendment must be upheld, despite private ownership.
Having the right to do something doesn’t make it a good idea. The issue here isn’t about Facebook having the right to do it.
The question was: "Who defines unreasonable"?
As a matter of fact, the answer is: Facebook. That _does_ seem like the issue to me.
I think that you're moving the goalpost from "who decides" to "is it a good idea". To which I've already answered: yes, it has a proven track record.
If you want something more generic, you could start with hate speech laws in Australia, Belgium, Canada...Spain, Switzerland, the UK, etc. Or even more generally, the UN's International Convention on the Elimination of All Forms of Racial Discrimination.
So, there's plenty of established frameworks to base your work off of and they've been around long enough that we can measure what the impact is.