Celebrities are human too. They can go enjoy music just like everyone else. This notion that celebrities are above normal people and fans have to faint and go crazy over seeing them is quickly dying, especially at these festivals. With the evolution of social media and celebrities being more personal and out there all the time, the "shock" factor of seeing a celebrity isn't as big as it was before. That is a big reason why I get frustrated when someone says artists from today aren't a big as previous ones. For example, people see all those videos from the Michael Jackson era of fans fainting, screaming, looking possessed, etc, and they think that is what a ICONIC artist is. Well, while that still happens a bit today, and it's mostly just because people want a photo to put on instagram, it will never be as extreme as it was in the past.
why do you think they're not asking Cher or Cyndi lauper? 
alternative indie music is NOT music that "flopped" 