Should a part timer get paid the same min guarantee as a full time pilot? Would they and should they benefit from the same CBA as a full time pilot? I think not. If airlines were allowed to use part time help, why would they bother hiring any more full time people who'd be represented by a union? Hypothetically, since the part timers aren't protected by the contract, they could be paid less and treated worse than the full timers. This would set a new precedent and devalue the worth of the profession. That's why it's shenanigans.

Doctors, lawyers, realtors, programmers and franchisees do not work part time as a hobby. They may work on a free lance, contractual basis, but it no case are they doing that kind of work part time for fun. In fact, all the fields you mentioned will easily consume your whole life and for most people they do. Except for realtor, I've seen all of the above first hand.