Do you make this demand in other aspects of your life? Do you check the ending of every book, every TV episode, every movie? Do you write to book publishers, movie and television studios and tell them they need to give proper warning because some people don't like not-happy endings?

To turn your argument around, just because some readers overreact to a fic, some of the time, does not mean all writers need to label all of their stories all of the time.