Why doesn't MiniDisplay out work with Thunderbolt-only Target Display Mode iMacs?
The display port standard only supports a video signal going in one direction (source to destination). The video card can output via display port, but  handling a bidirectional connection would require extra hardware to handle the inbound signal, and isn't part of the standard.

AFAIK the iMac's target display mode is done through the PCIe layer of thunderbolt, whereas the DisplayPort layer is only supported (by the graphics card) in the outgoing direction.