Here's the long detailed explanation, in case you're curious. This is an intentional change to Windows 8.1 in how background app management works.
Windows 8 was inherently designed in a way that you should never have to close your apps. The Process Lifetime Manager is a new feature in Windows 8 which automatically handles all that for you. Apps might temporarily continue to consume memory or can be suspended to disk, but only so they can be instantly resumed. If that memory or disk is ever needed for anything else, it's instantly freed up. Similarly, if those apps register for any background tasks like playing music, getting email, listening to push notifications, etc. they won't be closed.
It's similar to how smartphones manage tasks, especially iOS.
Still, people wanted to close apps. Why? There were really only two reasons that could be found during research:
1) The app was behaving badly, and the user wanted to close and reopen it to start "fresh"
2) The app was no longer necessary, was taking up room in the task switcher and back stack, and the user wanted to make it "go away" so it didn't show up there.
In Windows 8.1, #1 is handled by a heuristic. If you drag-down an app and immediately re-launch it, it'll start from a fresh load even though the drag-down didn't at first close the app
#2 still works fine.
So in the end, those things still work, while at the same time, in the event that someone dragged-down an app but later wanted to resume instantly where they left off, that works too.
