Valve ban advertising-based business models on Steam, no forced adverts like in mobile games
(www.gamingonlinux.com)
Everyone is acting like this is purely for good intentions, but I'll point out they make most of their money from taking a cut of the sale price from games. Ad money probably would not go to them at all. This is almost certainly purely a business decision, not because they fundamentally don't like the concept or want to protect you from it.