"Airfoil Speakers works pretty much like an AirPort Express from the point of view of Airfoil. It advertises its services over Bonjour, then uses the same AirTunes 2 protocol that Apple uses. However, despite using the same protocol, iTunes won’t talk to Airfoil Speakers. iTunes uses cryptographic authentication to ensure that it only talks to real AirPort Expresses, and we weren’t able to mimic that. Until Apple removes those checks, Airfoil Speakers will only work with Airfoil 3 and Airfoil for Windows."
IIRC, the traditional way around such legal ambiguity is for the project to offer a configuration variable akin to "input your favorite private key here." If the user is in a country which permits use of the AirPlay key, great, if not then the project can continue to operate as before. Either way, the project is indemnified because /they/ didn't provide the key.