I don't think it has anything to do with voice, safety, power of the device, nothing of that sort. What it is, is money.
Apple wants to capitalize from the turn by turn market, as it is a huge one. Just wait and see, I am willing to bet in the next few updates apple will introduce their own application for turn by turn directions, and make it the only one available on the iPhone for which you will pay a premium for, my guess is the 10-20 dollar range, possibly even an additional charge per month such as VZ navigator with Verizon.