I want to take back my earlier statement that a tile folder is like any other UI control that is simply waiting for input (and for which the Back key would close). It isn't that and I can prove it with a simple test: tap a folder, tap an icon to launch an app, back out of the app -- ta-dah, the folder is still open, showing that its "openness" wasn't a transient state (or that there are two bugs).
It seems reasonably clear (or at least consistent) that Microsoft doesn't see an open folder as being a control in a transient UI state. If it did, the folder would have to close when making a choice (to complete the in-progress operation)or pressing the Back key (to cancel the in-progress operation). Given that is behaves in a non-transient manner when completing the action (i.e., that the folder remains open even after a choice is made), the Back key shouldn't close it either.
I suspect that this debate is going to continue down the same path that Kid's Corner/PIN debate did. Essentially, we're now arguing about whether Microsoft mis-implemented the feature that they think users wanted or properly implemented a feature that users should have wanted.