Valve recently announced that it would change Steam’s policy to allow everything onto the platform, as long as they didn’t deem it illegal or “straight up trolling.” They said they want “to focus less on trying to police what should be on Steam.” That desire was spurred by a recent incident where Valve threatened to ban a handful of erotic games, and then actually did ban a school shooting simulator as well as a handful of other games.
Source: IGN.com Why Valve Can Curate Steam Without Stifling Creative Freedom