If you actually WANT to charge your iOS device you can use the middle connector with a regular Lighning cable using an Adapter that has the Resistor "code" to see every USB Source as charger...
(If it does NOT work like it is, such an adpater is << $1 from .cn)
But as was stated, most prefer their Tx not to be drained by the smartdevice...
Ender
Problem with this is the data lines from the reg usb on the mavic controller actually have a transmission signal... I would not use an adapter meant for chargers only.
Just speculating here, but from what I have read there is a handshake that occurs when you plug in your device to a usb port with active data line (mavic controller, pc, etc) and it will request a current draw. If say your laptop is not plugged in it may only provide a small amount of current at the time thus no charging is shown on an ipad but it will still get something none the less. Seeing as some people report having seen the charging symbol during use or for just a few seconds when connected, this might just be the reason. Who knows, DJI could send out firmware update to provide more juice when connected to an external power source, providing the circuitry inside can handle it, or just save that for next gen...