This may be strange, but I don't really understand, if she was happy with her ending, why she seemed to feel offended/felt the need to address it.
I mean, writing is an art, and enjoyment of art will always be subjective. There will always be a portion of people who hate your story no matter what, just like there will always be people who like your story no matter what.
If people want to rant about the ending, whatever, let them, their right and their loss if they didn't like it. (And I will probably be the first to rant about the ending.)
What struck me as a little off-color about the letter (other than what I stated earlier) is that she just seems really unfair to certain sects of her audience, like the audience that was sad they did not get a happy ending. It seems as though she is basically talking down at them like children. We know wars happen, we know we will someday be voting age, we know wars suck, um, hello, we read your books. We know happy endings aren't always true.
I personally like sad endings. Some people don't. No reason to fault them. Some people want to read happy endings because if they wanted a sad ending all they have to do is turn on CNN.