I imagine it is something to do with how much one could earn doing aava's as opposed to doing f/t work. Im full time atco doing pretty standard MM/AA/NN 4 day off. because of nights and limits on how long it takes till i can work again, the only time i can ever do aava's is on my 3rd day off or on a day i have leave booked.
However, if i was to go part time, i would get paid 2/3rds (i think) of my salary yet be able to make MORE money than a full timer overall because i have so much more time off where i am able to do aava's unrestricted by scratcoh... where f/t people cant. So i imagine its part of the agreement that if you want to go p/t, you cant do aava's as it would be unfair on everyone else who are restricted by scratcoh etc.
Thats what i imagine the reason behind it is.