I dunno. Thing is, she didn't ask them to publicise it, but they didn't ask her to take the band for granted and the fans for granted and blow them off to go to the cinema. If you're going to be a bit of a bitch, I don't think you're in any position to complain when your friends say "Oh, her. She's kind of a bitch."
Besides which, with the amount of loyalty the fans had to Tarja I kinda think they needed to go "look. this is why we're doing it" or else all hell would've broken loose. Moreso. I'd say what Nightwish did was fair and rational.
Although I fucking loved that they just stuck it on the end. "P.S. This is an open letter." Crowning moment of badassery, right there.
But yeah. I don't think there's anything unfair about it - she got her comeuppance, really. If you go around shafting people it's inevitable that you will get shafted back, and after the amount of dicking she reportedly did ("I know we've had this gig in the pipeline and it is SOLD OUT but i'd like to do a bit of rehearsing and then maybe go to the cinema?) she'd kind of earnt it. I think it'd've been a miscarriage of justice if they went around going "oh she's lovely." Or even "oh we're indifferent to her." Fair's fair.