I think the concept of musicians 'selling out' has always been silly at best. As for Metallica at Glastonbury -- how can it not be right? Two or so hours from the birthplace of metal...
Maybe Dolly Parton will join them for one of their songs...or vice versa. :D
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins