easy answer:
private license = you pay to fly
commercial = you are paid.
the problem is not only here , but in the responsibility of the CAA. the Airline, and the pilot paying.
what would happen in case of a crash? who is responsible?.
the copilot is paying to fly, he doesn't have the same responsibilities as a "real" copilot as he is still a "student", even with 450 hours on type.
he can go away with a : "I pay to fly, not my responsibility, go ask the CAA or my school"...
do passengers know that their pilots pay to fly???