I don't see how common carrier type regulation in the financial services industry could work. You're essentially forcing people to lend out or risk their money with anyone who asks regardless of their financial status or the risk profile of their business. Surely choosing what business risks you are willing to take must be some sort of right?
The only way out of that would be blanket government insurance for payment processors, but that would essentially be a massive subsidy and open to rampant abuses.
You do see that this is specifically an issue with the credit services threatening to refuse the processing of payments and not simply withholding credit. These are two different banking services, and you can support neutral payment processing without supporting neutral (forced in your vocabulary) lending. The reasons that the banks are as regulated as they are everywhere is because of the huge amounts of power that they have by controlling the flow of money. If you deregulated banks you would end up with one bank that controlled the world.
The only way out of that would be blanket government insurance for payment processors, but that would essentially be a massive subsidy and open to rampant abuses.