Earlier quoted context omitted.
Well, USB is a standard - you would expect a standard USB output to deliver a standard voltage.
Interestingly, even Samsung phone chargers put out 9v, but I think they only do that when connected to a Samsung device. On the plug is says 5v and 9v, and (I'm assuming) they switch to 9v when they detect a device that can use it for fast charging.
The charger emits 5V unless the device supports higher voltage and negotiates it.