There are a couple of issues here.

Fringers, if I may call them that, contribute to a festival in more ways than one. They will pay camping fees, if they are allowed, they provide free music to festival goers who are not otherwise engaged and they input into the local economy. They will also pay to see an act if they really want to.

I'm surprised that some people see them as parasitic. I'd guess that most festivals have a regular set of fringers and these will be enhanced by some of the paid performers on their time off. It enriches the experience of festival goers at no expense to the organisers.