Why do people consider any famous band "sell-outs"?

I agree, once a band becomes popular/famous, many people claim that they've become 'sell-outs'. They don't even use the term correctly, it's become a synonym for popular. I don't know why, I think they must feel special that they knew the band before they became big. But I understand it as, as a fan, shouldn't you want the band to grow and obtain a huge fanbase? It's kind of contradicting.