They're allowed to set their own Terms of Use.

I understand. My question is more about how much freedom they should have with their own ToS.

Let me explain why I think this question is important.

First, we all value freedom and in particular everyone should be allowed to offer business at his own terms.

However, payment providers are essential for online businesses.

This means the freedom of payment providers to refuse business collides with the freedom of other entities to do business /at all/.

IMHO, such situations should be regulated by laws to ensure a balance of forces.