[...] you can refuse to serve someone even if they’re in a protected group, but the refusal can’t be arbitrary and you can’t apply it to just one group of people.
To avoid being arbitrary, there must be a reason for refusing service and you must be consistent.
[...] you must apply your policy to everyone. For example, you can’t turn away a black person who’s not wearing a tie and then let in a tieless white man. You also can’t have a policy that sounds like it applies to everyone but really just excludes one particular group of people. So, for example, a policy against wearing headscarves in a restaurant would probably be discriminatory against Muslims.
[1] https://www.legalzoom.com/articles/the-right-to-refuse-servi...
Edit: formatting
So the missing piece of the puzzle is: which of the protected groups would every member of QAnon fall under? There's no law against discriminating against people who believe the QAnon conspiracy and spread it.
Ah, no, I didn’t quite spell that out.
They can get in trouble if their stated reason for refusing service doesn’t line up with the way they treat other people who behave the same way, IF that disparity in behaviour can help build an argument that the preponderance of evidence suggests that their real reason for refusing service was related to your membership in a protected class. They’re free to behave inconsistently in general.[1]
So if Twitter says they banned 17k QAnon accounts for coordinated misinformation campaigns but for whatever reason you could show that a bunch of other misinfo networks they knew about weren’t banned, and all the QAnon accounts were also known to twitter to be Unitarians, and this was part of a pattern of behaviour towards Unitarian accounts, that might be legally actionable.
But if there’s no underlying, unifying element to these 17k accounts that’s protected, Twitter is free to ban them from their service even if they’re not very consistent about enforcing their TOS.
[1] Which isn’t to say that behaving inconsistently is smart, precisely because it can open you up to these kinds of arguments if your inconsistency starts to look like a pattern of behaviour towards a protected class.