I think that creme fraiche is cultured, so that would explain using buttermilk. Adding lemon juice would sour the cream but that would be different from the cultured version.
I would have sprung for the real thing, too.
Letterman asked Zevon if his condition had taught him anything about life and death. ''How much you're supposed to enjoy every sandwich,'' Zevon answered. (From a 2003 NYTimes article on Zevon by Jon Pareles.)