Because it's not merely a "charging outlet" ... there's electronics in the charger to convert AC to USB power out :)
As I understand it these typically just use the power rails of the USB interface. I've used several AC->USB converters in the past, and they haven't been detected as a device. This is unusual at least in my experience.
I've yet to see a charger I've bought anywhere not give the warning the first time I plug into it