Well the decision on Google Maps was made when the iPhone first came out right? Before Android even existed, so before Apple even had any reason to fear Google. That doesn't explain why Apple hasn't now switched to something else though. Maybe cause Google Maps is just too good to leave out? I don't know.
What I do know is that Google Maps isn't attempting to commoditize smart phones like Google Voice is. So Apple has more reason to spite Google by rejecting Voice than Maps.