If you make a website selling used Seuss books and your bank cancels you I'll support you.

If you make a social media site that organizes domestic terror and your bank cancels you then that's your problem.

"If you make a social media site that organises domestic terror"

You do realise that most of the organisation of the "domestic terror" was done on Facebook right ?

How about making a website selling banned, unpopular or borderline racist literature? And then payment providers cancel services to it. Can I then take orders using cash via letters? Seems like it can work. Hopefully the post office, Fedex and UPS and other mail carriers don't notice and cancel it for some obscure money-laundering law/regulation about mailing money! Must I maybe resort to selling that literature on the street corner, back alley maybe?

What you describe occurs in China with books that might criticize their government, only they can't even sell them in back alleys. I've never heard of it happening with any material in the US short of things that encourage acts of terror. And I think that is the right place to draw the line.