Why Do They Care?

Zach and I have identified the tendency among the HPers of changing the meaning of words before, but I've never seen them as dedicated and inflexible about the term "anti-Semitism". We've tlalked about this in the past, but it came up again in a recent thread, so I'll repost the comment just so we have a current example:

DC
Why would anti Jewish sentiment or acts be called "Antisemitism?"
It seems a distortion, perversion of language, logic and reality.
To refer to people who identify with an ancient religious faith, who are from all over the world, of many groups as Semites.
With this logic, all people who identify with Abrahamic religions "Semites."
 
It is clear DC feels very strongly about the definition of anti-Semitism, enough that he'll call it a "perversion of language". I challenged him on it, telling him to read a dictionary, and he said that it doesn't matter what the dictionary said, etymologically the word should apply to all Semites, as in, Jews, Muslims and Christians. So this discussion isn't a matter of what the definition of anti-Semitism is, it's a matter of what the definition should be.
 
We can (and do) argue all day with these people who want to change the English language, but the question I want to pose is "Why do they care what the definition of anti-Semitism is?" We have a word for anti-Muslim hatred, "Islamophobia", and while Christianity is the dominant religion in most first-world countries, I don't think we'll have to worry about hatred for Christians. So we don't need to find a word to apply for hatred of the other two religious groups.
 
DC is not, presumably, a linguist. He has not dedicated any of his other posts to exploring the definition of other words and challenging them. He has not demanded, say, that the word "faggot", which could mean a gay person, an old person, a cigarette, or a bundle of sticks, just have one definition. Even if he was a linguist, he would pose his question in an academic paper, not in a comments section on the Internet. Why is only one word, "anti-Semitism", being singled out among all the words with nonsensical definitions?
 
It seems to me that this is just one more weapon in the anti-Zionist or anti-Semitic arsenal. They say that Jews aren't hated, and that hatred of Jews can't exist. What else are these people going to try to take away from Jews? They're trying to take the Jewish state, the concept of a Jewish nation, even the words in the English language about Jews. When does it end?