It's sort of a common courtesy, what w/ johncharles' points out about making known whatever one's tacit or published standards may be. Doing so would likely create a lot of self selection. If many performers back away one may make invitations, rethink the standard or let well enough alone if the public is happily paying.

My mother used to perform at the now-gone Rasputin's in Ottawa. The owner there had a clear plan and standard for each night of the week. I don't recall exactly which was which night and may have some of the details wrong but essentially Mon-Thurs were one of (not in this order):
...celtic jam, not sure how this one went or the skill levels.
...everyone-gets-a-turn song circle (jam at the option of each singer)
...IIRC, perform by invitation the shorter of 3 songs or 10 minutes
...wide open sign-up-to-perform (I don't recall how much jamming if any)

Fri-Sat had pretty polished hired acts with fairly polished warm-up act(s). Sundays, well memory fails me somewhat but it was something like 30 minute sets by invitation maybe a small cover charge went to the performers.

In that venue, with that owner's personality & rules of etiquette, performers self-selected pretty well, and audiences knew what to expect.