The reason is obvious – Apple wants to make it even easier to listen to that music anywhere you have an Apple device so you buy more music. From Apple. To play on Apple products. Have an Apple product? Great! Any music you can buy in iTunes will be available to stream over WiFi or 3G to any Apple device.
Interesting analysis of the fabled iCloud from Patrick, over at Minimal Mac. His point is so brilliantly simple and it makes so much sense that he just might be on to something.