Zapro:
And how should it be able to blow up your Arduino?
Powering through the USB is subjecting the microcontroller directly to whatever the power supply dishes out. If the voltage happens to be off or dirty I think that could definitely let out the magic smoke. I've actually never run into that problem despite powering some of my Arduinos with bargain priced eBay power supplies. Supposedly some of these cheapo power supplies are putting out lethal voltages! I think an Apple power supply should be good quality so not a concern in this case.