It's easy to say that an artist gets paid in terms of their ticket price, and yes a lot of/many/most people (myself included, if I had any musical talent whatsoever) would very happily accept that "fee", but if the artist is trying to make a living out of their musical career rather than have a free week(end) at Glastonbury with their mates then they are likely to be on tour, hiring transport equipment and possibly session musicians, so leaving all of that parked up in a field for five days just isn't going to be financially viable.
I'm not saying that what Glastonbury/WG does is wrong, and I don't know what the answer is, if they're even needs to be an answer, just my two cents. Also, for what it's worth, I was at that Summer Camp gig, and I'm a relatively big fan I suppose for our how popular the band is/was. They put on a great show to, I would have to guess after all this time, a few hundred people. Not sure if their career did fizzle out, or if they pursued other avenues - I think Elizabeth made a film. Would like to see them live again.