But would you be giving them a Christmas present if they weren't teaching your child? Isn't it directly connected to the fact that they are educating your child? You don't give a gift to the teacher two districts away because they aren't educating your child.
No, I wouldn't be giving them a gift if they weren't teaching my child. That still doesn't make it related to his education. It doesn't change anything about his education. It does nothing for his education, except in the examples I listed earlier.
I agree that the gift really doesn't have anything to do with the kid's education, though I may not be thinking along the same lines that SiotehCat is. The gift is related to the teacher's job, but it has absolutely to do with the kid's education unless the gift reflects something he or she learned in school.
Let's say my kid and I give her third-grade science teacher a Starbuck's gift certificate at the end of the year. That has nothing to do with the education she received but is merely a nice thank-you gesture. Now let's say that my kid and I make a cake for the teacher that is decorated to look like Saturn and put a bunch of appropriately sized cupcakes around it representing Saturn's moons because she learned about Saturn in science class. That gift has to do with her education.
It's a fine line and probably not terribly relevant, but it was bugging me.

In any case, I have no problem with what the OP did and I think Complaining Parent is one of the many, many people these days who are just looking for offense. Granted I don't have children, but I would be delighted to received a thank-you note like that and appreciate the time the teacher took to write and send it. It would be a great example for the kid to follow, as well.