Per spec USB2 output max 500mA per port. This is enough to charge an iPad when it's asleep, but as soon as the screen is on it will stop charging to power itself.
The wall charger and Macs include a non-standard trick (a resistor on the data pins in the wall charger, probably some active negotiation on Macs) to output up to two full amps.
Higher charge rates are part of the USB3 spec, but this would require both devices to implement it, and I don't know if it's even a mandatory part of the spec.
So, plug your iPad on a PC and it will charge, just slower, and only when sleeping.
The wall charger and Macs include a non-standard trick (a resistor on the data pins in the wall charger, probably some active negotiation on Macs) to output up to two full amps.
Higher charge rates are part of the USB3 spec, but this would require both devices to implement it, and I don't know if it's even a mandatory part of the spec.
So, plug your iPad on a PC and it will charge, just slower, and only when sleeping.