If this action was completely unprompted, sure, but I think we're talking about payment processors choosing not to associate with people who have already been the subject of boycotts on their previous platform.
A better example would be WikiLeaks which enjoyed a fair amount of public support at the time Visa etc declined to process their payments, as we have reasons to suspect that the pressure on Visa to do this may have been from the U.S. Government.
By contrast, there have already been numerous public protests against people hosted on the SubscribeStar platform.
I'm pretty sure that if you're in position of quasi-monopoly, it should be illegal
See: AT&T after "we" did, Microsoft after "we" didn't.
Apparently we do, otherwise they wouldn’t be doing it.
What I'm asking is if we as a public really want our payment processors to be making these moral judgements? That doesn't mean I think they should be compelled to do anything, but I certainly think it's cause for concern, and call for an alternative that IS willing to behave as a neutral party. I don't want the noise of what is effectively a pipe to be making arbitrary political judgements when deciding what is allowed to pass through.
Now, publicly funded banks, that's a different question. I absolutely expect neutrality within what is legal from our government institutions.