what a load of bullshit.
the penguin book of english folksongs contains folk songs not yesterday or yellow submarine or buddy holly.
if people want to sing buddy holly songs in an acoustic manner why not be honest and call it an acoustic music club, if you want to sing jazz call it a jazz club, if you want to sing bluebeat call it a bluebeat club.
if you want to sing unaccompanied only, call it an unaccompanied singing club, if its an instrumental session call it that.
if people do not know what to expect they will not turn up
if Martin Carthy is advertised you do not expect   Johnny Rotten, or Garth Brooks