In the past Valve has been the top company for allowing people to make money off of their games. In fact, based on content and messaging they've put out I'd say that it's a core principle for them as a company.
I don't think we're seeing the whole picture here.
Seem like Valve began to change. They used to be way more friendly with mods developer breaking ToS. I guess they smelled a great opportunity to take a cut of the cake
They're not really getting much of a cut like this though. It might drive some players back towards the official store but I doubt the small amount of money there is even remotely worth it. I kinda wonder if there's a legal reason for this, such as needing to legally regulate transactions and gambling within their game. As is, I know some custom servers have lootboxes, for example, which could get Valve in trouble. Otherwise, it seems very out of character for the company that has previously tried to endorse paid mods (albiet in a way that gives themselves a direct cut) and is normally very laissez-faire.