One of the biggest problems with Steam and Steam games is that handling disruptive players has traditionally been bit difficult. Valve’s anti-cheat software is good at catching and expelling cheaters, but does nothing to get rid of people who play by the mechanical rules, but not the social ones. Valve has decided to fix this by giving more power to game developers to ban the players they choose. But they still have to go through Valve to do it.
“Game developers inform Valve when a disruptive player has been detected in their game, and Valve applies the game ban to the account. The game developer is solely responsible for the decision to apply a game ban,” Valve said. “Valve only enforces the game ban as instructed by the game developer.”
Players who were banned cannot contact Valve to get unbanned. Instead, they must contact the developer themselves. This allows developers to institute whatever rules they like for banning players. Heck, they could ban you simply because they don’t like your face, although I doubt anyone would do that. You have a perfectly nice face.
Also, Valve needs to approve a developer before they are allowed to use this functionality, and if they are found abusing the functionality, Valve has the right to revoke a developer’s access to it.
Source: Valve