Clubs that don't have a cover usually pay a small, flat fee to the bands. Maybe there's a tip jar but the musicians aren't really making much money. I think it sends a bad message out to people that they can see the music for free. Having a cover charge helps to get people to take musicians seriously. I don't think it would hurt in some of the smaller clubs that don't currently charge any cover, to charge a minimum cover and give that extra to the bands.
People pay it all the time for two hours of entertainment.
Any event you go to charges a price to get in. It's an expensive proposition being a musician. People think you just show up and get to play. They don't take in to consideration that we have to rehearse long hours for free, we don't get paid vacations or medical and dental coverage and you put out, for the most part, your own promotion. So many musicians have day jobs as well, just to make ends meet.