Interesting discussion - I personally don't see too much of an equivalence between an open-mic / jam night and a covers band pub gig, though?
Open mic/ jam night: organiser has venue hire costs, typically provides all the PA equipment and lights, performers will be a pot luck standard and typically have around a 15 minute slot, give or take, and audiences will be chilled about the quality of the music ?
Pub covers gig - typically 2 hour performance time, band provides and sets up PA and lights and packs down at the end of the night and audiences expect a decent standard; if you don't come up to scratch you don't get invited back. Can involve a couple of hours of travel to and from the venue and another couple loading / unloading kit and setting up / packing down. Seems entirely fair to be paid for the time and effort involved in being able to deliver all that? Sure it can be a really fun night out, but any reason a pub should expect the musicians to provide all of that for free just because they enjoy the work?
I note you excluded function bands from playing for free, but many function bands will also be doing pub gigs and my experience is that pub venues are keen to book them precisely because they are a good enough standard to be doing functions.