Why do I have to cancel and then repurchase my pre-order to get the new singles?

Like so many other people, I was having problems getting the single from a pre-ordered album to download for me onto my computer. So I followed someone's advice to cancel my pre-order and reorder the album to get the newest single. Honestly, that is crap in my opinion. Given how much Apple/iTunes gets a year in album purchases, I don't think it's too much to ask to have the program work properly to automatically download new singles as they become available. How can this be fixed so that I do not have to deal with this every time I pre-order an album?