Re: What a load of old virginia
"USB devices are supposed to negotiate the current they draw"
You're right, but there are "strings" attached to that requirement. Power draw, especially from USB2, can be highly non-standard.
I've seen USB host designs that don't negotiate current, it's suppilied raw from the local 5v rail. (yep!)
I have a new-ish USB hub that of its 7 ports, has 2 ports that do NOT negotiate current - they pump out up to an amp each without discussion.
iThings are a little different - they're especially non-standard. To make sure the power supply can actually supply what the phone/tablet can ask for, there is a backward compatible kludge with resistive voltage dividers that the device monitors, and senses if it is indeed an Apple-qualified charger, and with what current capability. This way, the device can know what the charger can pump out, and do it cheaply so you don't need USB 'smarts'.
Next step up is USB 3, where it can negotiate higher voltages to get more power to the device. In this case, smarts are always used, because accidently pumping 15v+ into a device designed for 5v is obviously catastrophic.