If they did empty ferry flights.
I know of one Part 121 Chief Pilot that decided there was nothing against the Captain putting other company employees -- not required for the flight -- on board a Part 91 repositioning flight.
His requirements were that as long as they were on the Company manifest... and that they were employees.... you could stuff the aircraft full for all he cared.
(Mainly because he couldn't find a rule for or against it (at the time) in the company's ops manual.)
I believe the direct quote was, "I don't care who you put on board, as long as you list them all so I know who to notify later if you crash."
I (as a ramp-rat headed back home) and another dead-heading pilot -- took advantage of the situation, and we weren't stuck in Houston overnight.
(In fact the dead-heading pilot paid the fuelers $20 not to fuel us so he didn't have to stay in Houston another night. He did a turn out and back to Hobby from Intercontinental and then sheepishly climbed aboard as the fuel truck had just finished putting our gas on board. Ops gave it away when they started laughing when the Captain asked for the third time... "Where's that fuel truck?")
So anyway... The idea of "empty" ferry flights may not be exactly accurate.
And it was so damn cool to sit in the jump seat... and uhhh... to "allow" the FO to leave to go to the restroom and keep his seat warm while he was gone. I may have accidentally "manipulated the controls" a bit too. But only by accident.
(And the FO said my hand-flying sucked. He preferred the auto-pilot. He was in the "restroom" for a really long time. Like an hour or two.)
This was all very many years ago, pre-9/11, pre- locked cockpits with secret squirrel handshakes and what-not, of course.