You’re right, I didn’t take account of the possibility that they can cancel your contract if you have anything “illegal” on your platform. I was only relating to MasterCards statement:

At the same time, we require merchants to have appropriate controls to ensure Mastercard cards cannot be used for unlawful purchases, including illegal adult content.

in context of “you have to be 18 or older to buy it”. Juristic semantics can change that.

If you need to be 18 to own a credit card, then you’re old enough to buy those games. And buying with steam gift card doesn’t involve MasterCard or Visa for that payment, so it’s literally none of their business.