[Riddy]

Because, Rockstar haven't directly said, "You are allowed to run a 'mod' on our game" to the developer (Kalcor). DayZ has been 'bought' or now is owned due to Rocket passing it over by Bohemia Interactive, and DayZ belongs to a game, which allows mods to be created, and DayZ proved to be the most successful.

If the SAMP dev. got actual legal permission to run SA-MP from Rockstar then, Steam would allow it. Other than that, Rockstar can shut SAMP down if they wanted to.