I think you'll see that with every music download service. It's not a decision by Amazon or Apple, it's part of the licensing deal with the record labels. I used to DJ and as I started to move away from vinyl I started to notice that several of the older pop tracks I wanted to pay to download were only available if you bought the entire album. In some cases you could even buy every OTHER track on the album individually but not the one you really wanted. My theory is that the labels are trying to maximize revenue for one-hit-wonders.