HTC is committed to listening to users and delivering customer satisfaction. Since announcing our commitment to unlockable bootloaders, HTC has worked to enable our customers to unlock the bootloader on more than 45 devices over the past six months. In some cases, however, restrictions prevent certain devices from participating in our bootloader unlocking program. Rest assured, HTC is committed to assisting developers in unlocking bootloaders for HTC devices and we'll continue to unlock additional devices in the future."
Emphasis ours above. It seems pretty clear that AT&T is behind the "restrictions" here. It's unfortunate that AT&T is blocking an unlock for the device that many consider the best Android smartphone on the market today. We'd like to say that HTC will be able to push back and provide a bootloader solution, but we're guessing that despite the company's recent resurgence, it doesn't have much power in this relationship.
Of course, the fine people in the custom Android ROM community found ways to unlock bootloaders and provide custom ROMs before HTC began providing its (somewhat convoluted) online tool, and we're sure that they will in the future.