Even then, some "charity" events have substantial budgets for entertainment. Personally, my single best-paying gig every year is to run sound at a charity event. Staging, sound, lights, fencing, security, medics, porta-potties, dumpsters, etc. all get paid for. The suppliers of those products & services would laugh if they were asked to provide them gratis - & quickly go broke if they actually did. Yet the primary providers of the entertainment that contributes to the draw, that gets people to show up & drop their money in the buckets, are expected to donate?
Personally, I have enough respect for myself & my services that I expect to be paid more than the porta-potties.