actually if they were it would be easier to argue it's a violation of free speech, but since they're not they can censor with much less restriction
tbh there's a case to be made that the government should run a payment processor as critical infrastructure
If they can make laws and people are subject to those laws, then they are government.
I only disagree with the "the" on the GP, they are para-governamental.
The way governments these days use corporations and NGOs to work around constitutional restrictions, I'm sure they're pretty happy leaving the payment processors as-is.