It's not about how he expresses himself, it's the actions he takes actively trying to deny rights and privleges to others.
And the action of him writing this Superman story was going to deny rights and privledges of others. You may as well assault him while he walks down the street; that action of living holds the same offense.
It's about appropriate response. You are denying the man the right to live because he once supported something you don't like.