

As of July 16, Steam’s new guidelines state that game publishers should avoid releasing titles that may violate the terms and conditions of its payment processors. In other words, the storefront is asking creators to not only follow the platform’s rules but also submit to potential oversight from companies like MasterCard, Visa, and PayPal.
and from the petition
MasterCard and Visa have increasingly used their financial control to pressure platforms into censoring legal fictional content
Steam is enforcing MasterCard’s, Visa’s, and PayPal’s policies. From Steam’s Rules and Policies:
What you shouldn’t publish on Steam: … 15. Content that may violate the rules and standards set forth by Steam’s payment processors and related card networks and banks, or internet network providers. In particular, certain kinds of adult only content.
Point number 15 was not there in a Snapshot from February on the wayback machine. If anything, the solution should just be to remove the payment method for those games (which would still hurt the creators substantially).
There is a line that is confusing:
In response to this censorship, some fans have launched a petition on Change.org urging Valve to revert its policies
There may be petitions about reverting Valve’s policy, but it’s not the main petition against Visa and MasterCard (which is the one they linked).
No, that is not how that would work. People cannot buy games that violate MasterCard’s and Visa’s policies using MasterCard or Visa. If someone buys the game using a different payment method, crypto or a direct bank link, it would not violate MasterCard or Visa’s policies because they had no part of the transaction.
Being mad at Valve is reasonable, because they did not have to ban all games that their payment processors disagree with. They would need to remove the option to pay with those for certain games, and the process of filtering them out and deciding would take a lot of time, money, and labor. It’s easier for valve to just ban it outright, but it is not the right thing to do. Valve is not the reason it started, but there is reason to be mad at Valve as well.