USB chargers are not all the same.
"Modern" Adapters that implement the USB Battery Charging Spec, short the middle two data wires (D- and D+), the device can use that to detect that it can draw up to 1.5A (instead of the 0.5A that you get from a regular port).
Apple instead using a different (non-standard) scheme where they communicate to the power supply using voltages, and where different voltages represent different Amperage (I don't know the exact details).
So power supplies sold for Apple devices, as far as I have seen, do not short the data wires and as such a non-Apple device will not draw more then 0.5A
Just to add to this, because apple devices communicates with the charger over the two data wires using voltages, while devices that implement the USB Battery Charging Spec require the data lines to be shorted, the two implementations are incompatible.
What you could probably do, is crack open a 2A apple compatible charger and just short the D- and D+ and have a charger that will charge non-apple devices up to 1.5A. And it should also be possible to put a switch on the charger to switch between Standard and Apple charging mode. But if you mess up, don't come and complain to me!
Here is a nice video on how the apple charging works for the iPhone on youtube. But unfortunately this crap forum software won't allow me to post outside links.
Just do a google search for "reverse engineering apple charger"