I am pleased to be asked to sing anywhere where people listen to music and appreciate traditional songs.
I can do an evening unaccompanied or with accompaniment, but it does mean i have to practise songs differently, no bad thing in itself.
The rules are clear, so a performer does not have to accept the gig if they do not want to.
I am pleased that people organise venues and events that give people an opportunity to sing material that I like, yes, it is proscripitve.
Initially , it could be argued that an evening with musical instruments would provide more variety, but this is not necessarily the case,if the evning is mcd well and the organisers know his singers and the kind of songs they sing, he she can ensure variety by careful choice of order of singers