Why do red heads get offended if people call them ginger?

One reason could be that just as people do not like to be constantly called out by their skin color, red haired people do not like to be constantly called out for their hair color. They are people, and have names, and instead of addressing them as if they are their hair, it'd be best to address them for who they are; a person.

However, not all red haired people get offended when someone uses the Word ginger.