To expand on my reasoning:
I suspect the deliberate choice to use 256kbps instead of lossless delivery is a feature, not a bug, to enhance buffering for multi room playback, especially considering many consumers have really poor WiFi networks. If the company standpoint is that lossless superiority is not audible to the 98% majority, any means to reduce support calls about audio breaking up is advantageous. That actually makes perfect logical sense if one assumes quality degradation is inaudible, improved functionality becomes the primary factor that deserves optimization (by decreasing bitrate).
In WWDC 2017 it was presented that AirPlay2 enhanced buffering allows you to take out the trash (leave the WiFi network) without music being interrupted. At 256kbps I calculate this to allow for several minutes. With lossless compression, this duration may roughly become one third only, which would not be sufficient for taking out the trash. Audiophiles may ask why would they would want to take out the trash when they are enjoying the music, if you need to ask this question, this feature is not meant for you. 
Again I stress that this is my speculation only.