Push back and ATC
Around about 1984 'clear pushback' was changed to 'pushback approved'.
The reasons were:
1. ATC has no authority on the apron
2. ICAO did it that way
3. ATC could no longer see all the apron area.
The 'Approval' is for the aircraft to push in order to access the taxiway, not to pushback because it is clear behind. Separation on the apron is the responsibility of the wing-walkers, tugs etc. Over the years ATC has bought in by providing ad-hoc traffic information, but no clearance exists, or is permissible.