They want to manage their unsold inventory. Inventory age is a huge metric that most car companies concern themselves with; if Mustang sales are slowing then it makes sense for Ford to stop making them to let the already built supply sell through a bit.
So I just called FP to try and track down my order a bit, and the rep on the phone said that the 17 Shelby wasn't being produced till October...I'mma assume that's completely wrong but hey, maybe he was confused with a GT500 :p
Still not scheduled :(