Legally they don't have any responsibility to these researchers. They can ban anyone. The argument is that they shouldn't ban these people. That's why they're getting a polite letter rather than any kind of legal action.
Ideally (another should) moral considerations would always apply. External shaming comes into play when you've managed to outrage the moral sense of enough people, or powerful enough people. Yes, this does ultimately come down to social approval, but mediated by whether you can make a moral argument that people respect or not.
With regard to every previous argument we've had on here about Facebook banning people, it comes down to approval or not of the content, conduct, or actors being banned. Only the most extreme people argue that you should be able to post goatse to Facebook (not actually illegal!) without them removing it.