Tossing around "free speech" in this case is kinda silly. The first amendment only applies to the government, not some company.
1. Free speech as in the US first amendment. This indeed is limited to the government.
2. Free speech as in the enlightenment ideal upon which western liberal societies are built.
It is usually obvious that people mean the second because it is the only one that is even relevant outside the US. Somehow the narrow-minded people who can not conceptualize that free speech is broader than the first definition think it is a big gotcha' to jump into conversations with this kind of "um achtually".
This is becoming tiresome.
This doesn't even make sense. If a corporation is a person, then 1A Freedom of Speech means that the government cannot restrict the corporations political speech.
The corporation is absolutely allowed to restrict their users free speech, including political speech, because A) the bill of rights only binds the government, not corporations and B) it would actually be against free speech to compell a private corporation to engage in speech it does not agree with.
Should you be forced to post political or sexual content that you disagree with on your accounts or on a wall at your house? Of course not. Similarly, if you start a business, you cannot be forced to post political or sexual content you disagree with. Your freedom of speech as a business is what matters here.
The idea that we have "speech anarchy" where all people can say anything they want and punish anyone who doesn't reproduce their speech is insanity.
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.