I expect at home, in their own streets, glasto folk probably do take their rubbish home or to a bin, but something obviously happens there that makes them behave differently. Perhaps there are no bins? perhaps because there is loads of litter they think, my little bit won't make any difference, perhaps because they know there will be clean up teams working after the event?
"I've paid £x to be here so I can do what I like."
There's perhaps a similar mentality in regular life with some people; "That's what I pay my council tax for."