(if my atpl memory is correct!).
It's not.
Parallells of latitude are curved (small circles). Great circles are straight, not the other way around.
Your postulation assumes that you have a true track readout, in which case following a meridian would have you fly 360T/180T instead of 090T/270T - equally simple. However, flying a constant track of 090T/270T requires a constant change of direction. Flying a constant track of 360T/180T does not.