This change in behavior is intentional by Microsoft.
New apps written for 8.1 aren't supposed to close when the back key is pressed from the first page of the app. OId apps still do, but they can be updated to remain 8.0 apps with the new behavior. Old apps that haven't been updated will still follow the old "quit on back" behavior.
The idea behind the change it to let the OS more fully manage memory. With the new behavior, when a user "re-launches" a suspected app, the OS can just move it to the screen since it's already in memory. If the user used lots of other apps before re-launching one, the OS may have actually unloaded the app from memory (to use the memory for one of the other apps) so the OS really does have to do a full app launch.
Users don't have to worry about apps sucking up battery life because WP doesn't give CPU to suspended apps. Users don't have to worry about keeping memory free for the next app to be launched because WP manages that automatically. If an app truly hangs, users can "kill" it from the task switching view (long-press the back key and tap the "X" button or drag the app downward).