I'm not privy to the details of how AirPlay is configured, but knowing Apple, they probably built the streaming part of the protocol atop their own proprietary software tech stack (because that's how they usually solve these issues) and anyone who doesn't pay-to-play (with the corresponding cost burden of confirming interoperability with the infrastructure Apple built) is left out in the cold. Evidence I have for this is that you can control an AirPlay device from anything running Apple software (including a Windows machine running iTunes)... But not Android. Quick Googling confirms that the on-the-wire AirPlay protocol is undocumented.

That's Apple's style and it probably works better for an application like this (there are a lot of problems for which dictatorship is the solution in the technical space... I wish we had a streaming service dictator to solve the Chromecast problem from the other direction).