From a Netflix spokesperson, as published by MacRumors:
We want to make sure our members have a great Netflix experience on any device they use. With AirPlay support rolling out to third-party devices, there isn’t a way for us to distinguish between devices (what is an Apple TV vs. what isn’t) or certify these experiences. Therefore, we have decided to discontinue Netflix AirPlay support to ensure our standard of quality for viewing is being met. Members can continue to access Netflix on the built-in app across Apple TV and other devices.
Translation: AirPlay coming to non-Apple devices makes it difficult for us to prevent piracy. At least that’s my interpretation.
But you can just add this to the list of reasons I’m happy that I canceled Netflix a couple years ago — alongside their decision to not be included in the TV app and their decision to remove in-app subscription purchases from their iOS and Apple TV apps.