[Joona]

Or compare it with the Just Cause 2 multiplayer mod, which requires you to own the game on Steam, and acts like a DLC.

http://store.steampowered.com/app/259080/

Edit: Before they got on Steam, they required you to log in on their site and the mod with Steam's OpenID system, which they could use to check if you owned the game or not.

[voltagex_]

And Avalanche welcomed them, instead of sending goons to their houses.

[Joona]

I think that's why they got on Steam.