I mean that they told Visa/Mastercard that the client Steam/Steam customers are selling/buying certain types of products. Now that they are aware of it, they can be pressed to do something about it. Conversely, if they weren't made aware about it, they could just go on with their business as usual.
Personally I think it's better for private entities to stay neutral and leave political decisions to the government. It's hard to stay neutral about things when you know things are happening, so when you inform a private entity about what people are doing with their services, you are turning that private entity into a political entity.
Fair, although it would be better if the banks had some equivalent of section 230 protection from having to do anything about what they're processing, maybe.
What's happened here is that someone has complained it's visible on the store at all.