Although we all agree that this is legal to do (and nobody is thinking about taking Patreon to court), there is a second, more subtle social issue at play. Taking it to the extreme, if you live in a company town earning company scrip to spend at the company store, when the company decides that they don't like what you're doing in their place of business (organizing a union, perhaps), getting kicked out is functionally identical to a governmental sanction. As a result, even though the law permits companies to engage in heavy-handed social hedge-trimming, in some cases it can have the same problems as the government censorship that we have already agreed is bad. It would be going too far to say that corporations have an inalienable moral right to police whatever they control.