The cut they take is just one of the claims they have against Valve. Some of the other ones which another comment mentioned seem like fair arguments against Valve. The whole forcing pricing parity so game devs can't offer the games for cheaper somewhere else and DLC from other platforms isn't compatible with the Steam game and vice versa. And again you can say other platforms are doing that and worse too but that doesn't mean you shouldn't also go after Valve for it. Just cause they're a private company and because of that aren't as profit driven as other companies doesn't mean they still are gonna do things like this to increase their profits and maintain their majority market share on PC games.