Call me paranoid maybe, but for me it's about security. Charging the thing is the only thing you HAVE to do in order to use it. I don't want the only non-optional thing I have to plug into my computer to have firmware-level access to the hardware. You can put some pretty nasty stuff on those public USB charging stations, or those third-party USB power adapters. I know it sounds silly, but where credit card skimmers were a problem in the 90's and 2000's, compromised USB chargers will be hidden in plain sight security issue in near future. iOS has decent protection against this (though not perfect), but Mac hardware so far has nothing to protect against this.