Why didnt Amber Benson except comming back to Buffy when Joss Wehdon asked her to in season 7?

According to Amber:

"The timing just didn't work out. I was getting ready to go to London to direct the animated version of The Ghosts of Albion, so it just wasn't to be. I know that Joss did not intend to offend anyone with Tara's death. He was truly furthering a story line and didn't realize what an impact Tara's death would have."

[link]

I heard that she also said that she didn't want Tara's character to be spoiled by playing The First. (Evil)