I noticed some people saying "Holiday" instead of "Thanksgiving," too. I think it comes from a basic misunderstanding of why people use "Happy Holidays."
Contrary to popular belief, I don't think it's the "p.c. idiots" who overuse "Happy Holidays." It's just the idiots who don't understand why it's used that misuse it. "Happy Holidays" is something to say when you're not sure what someone's religious background is.
If you know that the other person celebrates Christmas, I don't think any politically correct person would say that saying "Merry Christmas" is wrong. However, if you're not sure if the other person is Christian or if they celebrate Christmas, "Happy Holidays" is preferable. It's not a question of pissing anybody off; it's just more polite if you don't assume that the entire world is exactly like you.
People mess up "Happy Holidays" when they use it for Thanksgiving and New Year's, though, and when they say it to somebody with whom they just had a ten minute discussion about Christmas shopping.
(NOTE TO READERS IN RURAL AMERICA AND THE SOUTH: There are places in the U.S. where there are sizable non-Christian populations. If you live in a place that's 100% Christian, you don't need to worry about what greeting to use; however, please don't make fun of those of us who live in more diverse places, where workplaces made up of Jews, Hindus, Muslims, Christians, and others is a reality, and where walking around saying "Merry Christmas" to everybody would make you look like a complete moron.