I was going to post a longer version of what Rory just said. It's built into the agreement the WGA have negotiated with the film and TV industry. That's the power of collective bargaining.
Of course, writers can end up fighting each other for a share of those rights, but that's the next stage; the rights themselves exist.
Mark's a star with a solid track record of creating hit comics that become hit movies, so I'm sure he gets a better deal on top of the basic rights, as he should.