Earlier quoted context omitted.
The ratings are probably more meaningful for requiring that users go out of their way to rate. Make it too routine and the courtesy fours drown out the users with strong opinions. Edit for the downvoters: this is me speculating on how the ratings work in practice. Whether the system is fair or serves your own interests are both seperate issues.
I disagree. There have been several times when the 'Rate this app' modal box came up and I wanted to give a good rating for it but couldn't be bothered because it sent me out to the AppStore when what I really wanted to be doing was playing that game/using that app.
Look at it from Apple's perspective. They want at least some great apps to float to the top and terrible apps to be tarred and feathered, while bothering users as little as possible. They don't particularly care about giving every app an accurate rating or capturing every user's opinion. Thus, they don't have much reason to push users to rate more apps.