Let me explain it to the best of my ability: You a politican, I a lobbyist, third party's have the politican under chain. If I want you to fuck someone day up, I can tell that third party to blackmail you.
To summarized in one sentence: Itch.io use Visa/Master Card as their method of getting payed, and Collective Shout lobby Visa/Master card to force itch.io to fuck everyone else up.