What are the other ways to collect payments? Paypal? Nope [1]. Stripe? Nope [2].
See the quote I posted here about someone who tried:
https://news.ycombinator.com/item?id=15551817
Generally it's not rare to see websites restrict their content not because its illegal, but because it's impossible to collect payment for any of your content, if some of your content is deemed unacceptable by VISA/Mastercard, etc...
And patreon marketed itself to content producers specifically as not following the paypal/stripe example.
[1] https://www.paypal.com/us/selfhelp/article/what-is-paypal%E2...