Much has been said on the different models for the various online music stores. Here is my personal take.
Music and video are two very different mediums. Music occupies one sense: sound. Perhaps you could say it occupies touch also if you turn the music up loud enough, but sound is the primary sense. Video occupies two senses: sight and sound. As a result, there are certain activities where you can listen to music, or watch video at the same time, activities that occupy different senses.