Wishing People Happy ............ Holiday for Holidays They Don't Happen to Celebrate

I had an interesting professor in college. He was tough and not so much fun to talk to after class but still, he was a brilliant professor and knew his stuff. He taught one of my 19th century British poetry classes. One day I was discussing my paper with him after class. It was the last day of class before the Thanksgiving Holiday and so as I started to walk away, I wished him a Happy Thanksgiving and gave him a smile.

He gave me a deadpan look in return. Continued to stare me down. Then he gave a sort of slow shake of his head, turned around, and walked away.

Now, I know that my professor is of British origin. But he lives here. If I remember correctly, he's lived here for years. It is likely he has U.S. citizenship now. He has no plans anywhere in the near future to move away from the U.S. Why is it rude for me to wish him a Happy Thanksgiving? If I were in England and it was Guy Fawkes Day or anything else, I wouldn't get offended for someone telling me to enjoy the Holiday.

Am I missing something here?

I suppose the reason this really stumped me is because Thanksgiving is such a generic holiday to me. Yes, its origins lie in the first settlement at Plymouth and the first dinner with the Indians but it has evolved into a holiday that seems it can offend no one, with the possible exception of people suffering from eating disorders or those who literally do not like food. It's a day of Thanks. The holiday that it has become has very little to do with being American at all -- it's just about being thankful.... It's about spending time with family. And, unlike every other holiday including Easter, which is about the rising of Christ, it is the only one that is not overly-commercialized. I mean, yes, green bean, fried onions, turkey, and other food establishments of the like do do well this time of year because of mass food consumption, but I mean, come on, it's only dinner. There are no Thanksgiving cards, no Thanksgiving baskets with Thanksgiving decorated items. There are no Thanksgiving presents or Thanksgiving fireworks (which is a little sad)....

I'm writing this badly just to get it off my mind but seriously, did I miss something?