because some people have MAC computers instead of PC and they play in MAC version of MA.That version sucks badly and they even dont have patches installed on it.So they can't make challenge time limits and they cant auto-finish battles.
I just decline such challenges and send them my own with desired limit.
That's not entirely correct. I have played against several mac players and they and their challenges works like supposed.
Currently I have atleast 1 game going on against mac player and he challenged me and turn limit is 14 days.