2.1 Amp charger only charging as USB, not AC

[BlueWRXPride]

I just got the iGo MicroJuice car dual-usb charger. It is rated at 2.1 amps, not I'm not sure if that is for each port, or total between the two. Either way, I would assume there would be at least one amp per port, which should make my phone (HTC One X+) show that it's charging from AC, not USB. However, it shows up as USB. I tried using the cable that came with the phone, but it made no difference. When using this cable with the wall plug that came with the phone, it shows up as AC. I thought that if I got a car charger rated for at least 2.1 amps, I would be sure to have it charge fast with AC mode. What is stopping this from working?