I think it is obvious why valve doesn't want this. People are selling things in their game through a route they don't have control over. It might even get legal

I think it is obvious why valve doesn't want this. People are selling things in their game through a route they don't have control over. It might even get legally questionable. The title is also lying, at least the tweet only mentions one possible way of monetization. It explicitly calls out selling items, but not other forms of monetization, e.g. donations.