Its very likely that Apple has some legal agreements surrounding AirPlay technology as well as patent payments. There could very well be some agreement with music label about how iTunes music is sold and payed (especially the DRM versions). Allowing someone to reverse engineer their technology and violate legal agreements and/or patents is something they may not want. In the end, by controlling the ecosystem, Apple is in part responsible for what their devices are used for (remember the licensing fears with GrooveShark?).
