Take this as another analogy. Let's say you're trained as an electrical engineer. You go to work, some people start calling you a mechanical engineer. In general, wouldn't you correct them and say "no, I'm an electrical engineer"? Some may even be offended as there are people who consider mechnical engineering inferior to electrical engineering and would say something like "duh, can't you tell by what I'm working on that I'm an electrical engineer?"
Sure both are part of the engineering field, both are similar YET DISTINCT with many overlapping fields of interest, maybe even the same classes are required to be taken by both disciplines, but I think most would agree that they are not the same.
I think what it come down to is people want to be recognized for what they are. Every culture is unique, regardless of some shared experiences, physiology, or traits. When one gets combined into a larger classifcation, a lot of that uniqueness that makes a person or group who they are disappear.
You may not be offended when somebody lumps you into a larger category, but that doesn't mean that when somebody does get offended that they're "whack", "sensitive", "PC", or whatever. Shouldn't everybody be called what they want to be called? If your name is Bob and I elect to call you Stan, isn't that just me being an ass?