If a band is being used as an attraction to make money, then the band should receive a cut of that profit. Either a fixed fee or a share of the ticket sales.
Gigs can be expensive and hard work (especially if you have big amps) and there's a fair bit of boredom between sound check and gig.
I've read that some headline acts are now charging support bands to tour with them, I think it was Disturbed charging $12,000 to support them on their US tour.