What kind of "speech" are we talking about here? If a payment processor is already required to be secure, it could also be required not to deny any legal transactions. This isn't even political, you wouldn't expect a mobile carrier to censor your phone calls (at least in the EU we don't have that.. yet).
The concept that you're talking about in the US is a "common carrier" e.g. a taxi can't deny some people or a hotel can't refuse some people.
In the US, payment processors are not common carriers and operate on a contractual regime that allows them to refuse or terminate service for non-compliance, risk management, or policy reasons.
Mobile companies here are common carriers and are much more strictly regulated.