> But by and large, if you do or say something on my premises that I dislike, I can kick you out.
It cannot be a fuzzy "I dislike what you do", as it is prone to selective enforcement (and I can still discriminate based on what you are, but maintaining plausible deniability).
In reality what they would actually do would depend on what that "something" is. If it's something like "don't eat meat on odd-numbered dates", you're probably fine. On the other hand, you say something harmful or distasteful to other protected categories, you'd be lucky to keep your account. They'd weigh the harm to you of kicking you off versus the harm to others by letting you stay and (of course) the PR liability incurred by doing either.
If the discrimination criteria are broadly found to match up with a protected class you're in legal hot water. IOW using proxies for "plausible deniability" is a risky strategy and businesses have been punished in the past for doing it.
> It cannot be a fuzzy "I dislike what you do", as it is prone to selective enforcement
You're right it can be. But again, consider the alternative on online forums. Anyone could post porn or vulgarity on religious forums, or socialist propaganda on business forums. It would be very disruptive if selective enforcement weren't allowed.
You're missing the fine distinction between protected and non-protected categories. Discrimination based on what you are is banned - those constitute the protected categories. Sex, gender, national origin, race, family status, veteran status, religion, and sexual orientation all fall under things you are.
Business and private organizations however, are allowed to discriminate against their customers based on their actions. It gets fuzzier if those actions intersect with a protected category (e.g. a supermarket probably (IANAL) can't throw out a gay couple for family-friendly PDA if a hetero couple would be allowed to stay). But by and large, if you do or say something on my premises that I dislike, I can kick you out.
Imagine if weren't so. Anyone could disrupt religious services by shouting obscenities. You couldn't have a peaceful shopping trip or grocery run. It would be total chaos.