It ceased to be freedom of expression because it’s viewed via the prism of a highly sexist and racist capitalist system. If festivals just booked acts from the total pool of all acts then you’d be right, but they’re booked based on a metric that comes from who’s had the best management, label power, radio play and media push, which is very far removed from the notion of ‘art’.
For those guys the actual music being played is fairly low down the list and that’s fine. I’ve known someone who went to ATP in 2006 and saw the Yeah Yeah Yeahs set only. They had a great time.
And Some people just come for that one headline act because it’s the only way they can get to see them live. Again, this doesn’t stop people making the rest of the festival varied and interesting.