Valve's payment processors told Valve they would withdraw payment processing unless Valve banned specific categories of game from their online store.
The payment processors did not cite any law; Valve selling those games was not illegal. Instead they cited Mastercard's rules, which say that they cannot submit transactions that Mastercard believe might damage Mastercard's goodwill or reflect negatively on its brand. Those rules also say Mastercard has sole discretion as to what it considers breach these rules, and Mastercard gives a list of what it deems unacceptable:
https://www.mastercard.us/content/dam/public/mastercardcom/n...
> 5.12.7 Illegal or Brand-damaging Transactions
> A Merchant must not submit to its Acquirer, and a Customer must not submit to the Interchange System, any Transaction that is illegal, or in the sole discretion of the Corporation, may damage the goodwill of the Corporation or reflect negatively on the Marks.
> The Corporation considers any of the following activities to be in violation of this Rule:
> 2. The sale of a product or service, including an image, which is patently offensive and lacks serious artistic value (such as, by way of example and not limitation, images of nonconsensual sexual behavior, sexual exploitation of a minor, nonconsensual mutilation of a person or body part, and bestiality), or any other material that the Corporation deems unacceptable to sell in connection with a Mark.
The payment processors threatened Valve first. Mastercard doesn't need to threaten Valve or even contact them at all to force its will on them: it just needs to threaten its payment processors, the same outcome is achieved. Valve did not remove games from sale until threatened. If they did not do that, and instead initiated some kind of fightback, they would most likely find themselves completely removed from all payment processors, with no recourse. If you want to call that "precompliance", so be it.