While their makers surely don't condone unauthorized copying, a media player of this kind has no way to distinguish between an authorized sound file and an unauthorized one. The PRS tracks all media playback, and submits data to the relevant authorities, so that copyright holders are compensated properly. So, today's so-called media jukebox is not a contender for the title of PRS, although with a few improvements it could be. If such a device were offered as a hosted application, and support for live Webcasts were added, it would come pretty darn close to our Holy Radio Grail. So it's plain that the technical challenges of building the PRS are not that great.
Legal issues are what's keeping it in the bottle for the moment.
Webcasting Versus Audio on Demand To the copyright authorities, our PRS would come too close to audio on demand for comfort. Audio on demand, however, is a different kettle of fish. It may seem that there is little practical difference between the two, but to the copyright authorities, there is a big difference. Webcasts featuring both signed and unsigned talent are very common, as is audio on demand by unsigned artists. At the moment, you'll find very few sites where work by signed artists is legally available on demand.