The timeline is very important
After Rudolph and Hermey returned to Christmastown, everybody realized that they treated them badly. Even Santa and the head elf. Later Santa announced that Christmas was cancelled due to the storm. During his announcement, Rudolph’s nose nearly blinded Santa. It was then that Santa asked Rudolph to guide his sleigh. AFTER he and everyone else expressed regret for their behaviour.
So, no, everyone is not nice to Rudolph only because he is useful.