I don't know. Jack White may make sense. Guests "we can't tell you about." Maybe Jack White has a radius clause due to his Ryman shows, or a festival clause from his Hangout or Lollapalooza contract. And while he can't be on the lineup, maybe he will still show up and take the stage.
I see nothing wrong with booing them off the stage. Sure, they will be doing what they were booked to do, but what they do is cover another band's music. They have no artistic integrity, and thus deserve no deference.
Yeah, but why waste the time when there's other things going on? I'd rather see GZA than boo a band off stage any day