Interestingly, a state monopoly could actually be more tolerant on some types of business, because of the legalities associated. As an arm of the state, it's subject to the state's obligations for equal access and service.

If their policies were anything less than "we support any legal business", they'd be spending decades in the courts.

Conversely, a lot of private payment processors are beholden to the tastes of their clients and investors. Even if you have all the right metrics and tech about fraud, chargebacks, customer satisfaction, see how many processors close their doors if you say you're selling rifles or porn.

Ideally that would only happen if the business was illegal or found guilty of serious crimes. In those cases society has an interest in killing it.