> Is this right only enforcable in the negative?

Rights like speech, association, and assembly come in a positive and negative form. You have just as much of a right to not speak and not associate as you have to speak and assemble. For fairly obvious reasons the right to speak and the right to assemble gets invoked and litigated more often, but First Amendment lawyers typically have held that forcing parties to speak against their will is equally problematic.

In this case forcing AirBnB to take these people in would be to violate their right to free association. With a few exceptions[0] you are not forced to do business with anyone against your will. The idea that we would trample AirBnB's rights in order to make it easier for someone else to exercise their rights is more than a bit troubling.

> Are there cases where the government can disagree with business and can instead encourage assembly...

The government can encourage all they want. As free citizens you also have the right to give the government the finger and say no.

> or do private ToSes veto that?

Depends on the quality of the ToS. You can't just throw obviously illegal or coercive stuff in a ToS and expect a court to uphold it. But the government has regularly upheld the idea that contracts can contain restrictions that the government could not possibly do, at least as long as the contract is freely entered and well formed. The government might not be able to take away your right to speak, but you can absolutely enter into a NDA that will restrict your right to speak.

Similarly, it's legal to walk around without shoes and a shirt (well, if you're a man). But it's also legal for a business to declare "no shirt, no shoes, no service". This is a ToS "vetoeing" (bad term) someone's right to certain types of behavior within the confines of their own business.

> What regulations aren't authoritarian?

Are speed limits authoritarian? What about food safety ones? Most of us would say "no, those are not authoritarian".

Regulations become authoritarian as they begin to expand in scope and arbitrariness, and when the benefit offered to society does not outweigh the infringement of the regulated parties' rights.

Fundamentally, the government does not have a compelling interest in making it easy for a specific party to assemble and protest. They have an obligation to not make it arbitrarily hard, at least excluding reasonable public safety measures, but it's not their responsibility to ensure that there is adequate housing and transit. The idea that the government might get involved in private businesses to force them, against their will, to assist in a political protest is deeply alarming. Down that road bad things lay.

0 - The obvious exception to the right to not associate is anti-discrimination laws. Within living memory there have been various prejudices so common to effectively lock certain minorities completely out of participation in civic society. Congress and the federal government have persuaded the courts that the state has a compelling interest in ensuring that these minorities are able to participate in society, a compelling interest that outweighs the rights of business owners to decide who they want to do business with. These restrictions are largely based around immutable characteristics such as race, sex, or religion (it's assumed that religious beliefs are deeply held, and nobody can force you to change). This obviously excludes political affiliation, behavior, speech, etc, and therefore isn't relevant in this case.