Similar to the recent actions by Twitter, Google, Apple and Amazon; Visa and Mastercard are private companies and can do whatever they want. Anyone who has an issue with what Visa/MC did must also take issue with the others.
Yeah .... no. For a variety of reasons. But for one, it's impossible to do business online if you're not in the good graces of major payment providers (assuming you want to get paid). It might be hard(er) to advertise your business if Twitter bans you, but that's in no way comparable to how essential Visa/MC are to any business.
Providers of critical infrastructure (payments, search, appstores, probably a few other things), when they're big enough to be a monopoly/duopoly, could really use more oversight. If you piss off Google/Apple, your business can be ruined, and you'll have virtually no recourse -- as evidenced by the frequent "google/apple killed my app" threads on HN.