(Why also do they always always always roster you to start on an early shift after leave)
Because they can. According to the regulations, originally written by a pilot (with no legs), for the pilots, you can commence at 0600 local after a day off.
An airline cannot be criticized for adhering to the regulations, there are plenty around that don't adhere. A pilot expects the airline to adhere to the letter of law, well what's good for the goose .....