"Do you ask your plumber to pay for the right to work on YOUR plumbing? How about your dentist, must he pay for the right to play around inside YOUR mouth? These guys offer a service as does any entertainer."

Agreed but...

If I hire a plumber I expect to get someone with experience and expertise, someone who has made plumbing their life's work. Thus I expect to pay for that. I believe the same thing applies here. The venue was an open mike in a restaurant. By definition an open mike is a venue for amateurs, not professionals. The show can be just as good but these are not people who make their living with the music. The impetus is there, the talent may be there but they still have a day job. As such, I think they should expect to pay to play. Not much, but enough to support the venue and keep the open mike concept alive. It is similar to self publishing.