Actually there is no conspiracy or Jerk-ness about this at all. T-Mobile USA uses a pretty standard system for doing Wifi calling similar to what it does with VoLTE already. Its essentially the same traffic and is similar in principle to VoIP. Its all data going over a network. Its no longer separate from the data network aside from a higher priority being given to voice data traffic over regular data (QoS). Android ha supported VoIP for a while, HOWEVER Wifi calling ISNT VoIP. You need extra code and logic to handle routing and dynamic fail/rollover support mid call. As such until Android includes this code in the basic AOSP (open source) or Nexus/OEM (proprietary) code... those devices won't work with it unless said features and the user interface for it is added.
iOS with version 8 now has that extra code baked into the OS, thats why Wifi calling is suddenly possible on the iPhones. Its not magic or some direct deal, just now its included in the system files directly from the phone manufacturer. For Android currently you have the OEM customizations and Wifi calling is added additionally in the carrier customizations packages.
So what does this mean?
If Android L, much like iOS 8 now, includes it in the core OS then ALL Android L phones that are on the T-Mobile will get it. T-Mobile already has done the work on their back-end so all thats left is those software components on the phones to be added.
You can't expect Google to go into their Nexus 4.4 code just for T-Mobile. Its a big re-working of the networking stack to handle two independent IP networks at the same time.