Is it the freedom of banks to not offer checking accounts to minorities? Why can't a bank express itself in the form of denying service to some people? Must they be compelled to?
Yes. If these platforms and payment processors want to continue to enjoy the state-enforced privileges they receive, then they must, within reason, offer their services to all.
It's also not about protecting any opinions (popular or unpopular) from private refusal to assist in propagating them.
Free speech is not the same as anyone else being obligate to provide assistance with distributing your speech, in fact, such an entitlement would violate freedom of speech for those compelled to participate in speech they disagree with.