I think there's a fundamental flaw in thinking about the musicians "paying to play." They aren't paying to play. They're paying to get inside the building. If all they wanted to do was play, they could stand outside on a street corner or in the park (or wherever it's legal) and play to their heart's content.

Just give a moment's thought to the overhead involved in having a venue. There's rent or mortgage, furnishings, staff, insurance, utilities, etc. If the musicians find paying a cover charge to get in unacceptable, they're free to provide their own venue. See how the expense of doing that compares to paying a cover charge and/or minimum order in the restaurant.

I think there's also a supply and demand issue here. People are willing to pay to hear music, depending on who's playing, what they're playing, etc. But not all people are willing to pay, or pay much, to hear all music. How many volunteer plumbers or auto mechanics are fighting for the chance to fix your pipes or repair your car? How badly do you need those things taken care of? Now, how badly do you NEED to hear this music, and how many performers are scrambling for the chance to provide it? In most markets, people playing the type of music we're discussing get paid little or nothing because, ECONOMICALLY SPEAKING, it's worth little or nothing. It's probably doing well to get the patrons to pay the cover. There's simply much more of this music available than there is willingness to pay a lot to hear it.

Bottom line, if the musicians in question are providing such a valuable service, why aren't they actually being hired and paid well to do so? That probably sounds harsher than I mean it, but I think it's valid question. To get into a snit about paying a nominal fee to participate in an open mic venue is, in my opinion, demonstrating a failure to see the realities of the situation.