I remember playing as member of Local 802 of the AFM in NY and the union would determine how many musicians HAD to be hired for certain rooms, based on capacity ...
One of the first synth 'scandals' in my hometown was when one of the popular young band leaders started using backing tracks and hired non-musicians to stand in front of KBs and make believe they were playing ... of course the 'stand-ins' were paid about 1/3 what a musician would have made ...