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.
I think the main difference is that in CSGO (and Dota) cosmetics sold directly by Valve in the game can be regulated to follow the law in all countries its available in (IE, the changes made for France in these games). Valve does not have as much control over 3rd party servers and sites. If its entirely separate like more CS gambling sites, they can (and do) just distance themselves from it and ignore it in all but the most extreme cases. On servers and custom games though, its accessed directly through their game, which is why I can see them being more strict with it.