Of course he is within his rights. The right of parties to contract (or reject contracts) is one of the fundamental rights of civilization.
And, therefore, Fox is well within its rights to reject his offer.
But let's say Fox wants to offer merchandising rights. It's pretty obvious that, for example, for every Burns or Flanders t-shirt sold, probably a hundred (or more) Bart or Homer t-shirts were sold. Burns, Flanders and the rest of his characters are secondary characters. They aren't the merchandising cash cow that, for example "Bart Simpson Underachiever" or "Mmmm Beer" stuff is.
Therefore, there's little incentive for the studio to cut Shearer in on the merchandising. His characters and, therefore, his participation, adds relatively little value to that merchandising.
And, therefore, Fox is well within its rights to reject his offer.
But let's say Fox wants to offer merchandising rights. It's pretty obvious that, for example, for every Burns or Flanders t-shirt sold, probably a hundred (or more) Bart or Homer t-shirts were sold. Burns, Flanders and the rest of his characters are secondary characters. They aren't the merchandising cash cow that, for example "Bart Simpson Underachiever" or "Mmmm Beer" stuff is.
Therefore, there's little incentive for the studio to cut Shearer in on the merchandising. His characters and, therefore, his participation, adds relatively little value to that merchandising.