According to Wikipedia, that thing has a 6200 mAh battery - just about right, i.e. you'll need ~ten hours to charge that at 500 mA, which basicall is all that USB (2.0) is specified for in the best case.
Why it takes longer? No idea. One option may indeed be that HTC uses one of the various standardized, or even its own, protocol to communicate between charger and tablet before it enables higher charging currents. Another possibility might be that they may be using the more usual voltage sensing, but any one or several components in the chain (charger, cable, tablet, any of the connectors) is dropping a few mV more than absolutely necessary and therefore the tablet goes down to 500 mA.
BTW, this is not limited to your specific combination of tablet and charger. Plug in any arbitrary device to any arbitrary power supply, and you'll find more likely than not that will only charge at 500 mA. Add to that a large battery, and you'll end up with long charging times.