I know for at least some of them it's because they've developed a deep and abiding hatred for All Things Joss because He Ruined Buffy by Leaving It in the Hands of The Evil Marti Noxon for Firefly. *snort* Whatever.

Ah, yes, I love it when people do that, and then they also forget all the wonderful things that Marti Noxon did on the shows (she wrote one of my favorite lines of dialogue ever -- "God doesn't want you, but I still do." That line still gives me shivers, man.).