Well Thomas, I think that the organisers of a venue have the rights to decide how it goes in terms of whether backing/ others playing along is to be allowed or is part of the event and if an event is run in such a manner, it is up to the performers to fit in.

Having said that, in the events I have been involved with, it has always been the right of the performer(s) to decide whether they want extra backing and the "rule" has been "you don't join in unless asked to" and this has stood regardless of whether a song is being sung acapella or not. My view is that it is THEIR song/tune...

The instrumental sessions round here of course have the understanding that once a tune starts, it is an invitation for all to join in but once in a while, someone (maybe more than one person singing together) will want to sing a song. Again, the understanding is that is their song and the other musicians remain silent (or join in singing the chorus if it is that sort of song).

Jon