Would you say the same thing about a politician? Going to vote requires little effort, but their main function is to provide us with a service, if you didn't get that service, it would be fair to voice your complaints. Why should a band be any different? Supporting a band and allowing them the freedom to make music for a living - for our enjoyment - means that the fans should come before indulging their taste in support acts.
As for the Chili Peppers, I bought all their albums, shelled out for the gig and travelled to London to see them trot out a bland, soulless show. Flea's bitterness towards his audience seems, to me, misplaced considering the circumstances.
And you have to take your audience into consideration. The sort of crowd the Chili Peppers attract, for example, is not the sort of crowd that tend to go for garish, electronic lesbian bands, so it just didn't make sense to have them on the bill. James Brown also played that day and went down a storm, probably because he was a big influence on RHCP and stylistically, isn't a million miles away.
These days, as my taste in music has broadened considerably, I'd probably be a little more enthused by Chicks on Speed. Unfortunately, the frankly dreadful half-set they played did not endear them to me and I'm not sure I'd be able to give them a listen without being reminded of what a massive disappointment they were.