Correct me if I'm wrong, but Sarah has never shown up drunk on set, never shutdown production due to "personal issues" (AKA rehab), never bad mouthed the show or her co-workers (I don't count Freddie's most recent bout of stupidity), and has seemingly always given 100% to the show. She did everything she was contractually obligated to do. Doing press for the show, participating in Cons, or going to wrap parties, to my knowledge, is not something she is obligated to participate in.
Now, that's not to say I agree with her decision. I do feel it was the right thing to do to be there. To show her appreciation for the cast and crew as well as what this show has done for her. It's entirely possible that without Buffy she may still be relegated to soaps. It's also possible she'll suffer from the same backlash Harrison Ford did for refusing to discuss what launched his career, Star Wars, and have to live with it.
But, I also know there are celebrities, as well as non-celebrities, that take a hard line to separate work from their non-work life. I work in a large firm and know people who have never participated in any social or outside function. Granted, they may not be seen as the friendliest folk here, but that doesn't make them bad people either.
It just seems odd to me that so many people seem to be getting their knickers in a twist over this. It doesn't change the character of Buffy nor the outcome of the show. Only if she had chosen to pull a David Caruso would I have been quite put out.