Technically this is because that specification referred the two data lines are short-circuited and the phone thus instead of a USB port detects a power supply. HTC plays it safe and includes the data lines not only in the short adapter, but also in the cable. For me, this is doubly useful as this one hand the HTC adapter with * any * USB cable is detected as a power supply, and on the other hand my old Belkin adapter when using the HTC cable also, so I this now with the Desire can be used for navigation.
(Side-effect, but meaningless for this application: The cable is not suited as a data cable.)
I am extremely happy with the product. At the delivery by the mobile-Punkt shop, there was also nothing to complain about.