Yeah. I think at&t allows unlocked devices to use Enhanced LTE services but it looks like they have to be certified. This means, and i'm guessing, it's been left up to the manufactures to have their unlocked devices approved by at&t before they have access to enhanced LTE services. HTC decided to jump the hoops. Still wish at&t had an open door policy with E-LTE like TMo.
HTC has done this with the A9. Quote from the network compatibility page from HTC. See Below.
"Unlocked devices usually don't support services such as Voice over LTE (VoLTE), Wi-Fi Calling, Mobile Hotspot and LTE Carrier Aggregation. Does the HTC One A9 Unlocked edition support those services?
Yes, unlike other mobile phone manufacturers, HTC is proud to offer the best user experience to our customers. We support Voice Services over LTE (VoLTE), Wi-Fi Calling, Mobile Hotspot and Carrier Aggregation. Our HTC One A9 has been built from the ground up for the North America customers and wireless networks."
I spoke with an owner of an unlocked a9 on at&t they they confirmed they have Enhanced LTE services and VoLTE.