I think you are wrong, It's not actually an inconsistent in design. By design if you press the back key on the home screen, it actually scrolls straight to the top tiles.
[First, apologies for the crazy length of this.]
I started to compose the same kind of message, but then I realized that both you and Homeric are right, but about different things.
You are right that pressing Back when the back stack is empty goes to the top of the Start screen. It has done that at least since 8.0 and I think that it did as far back as 7.0.
I think that Homeric is wrong in saying that pressing Back is supposed to un-expand controls, as it does in the week view of the Calendar app after you've tapped a day to expand it.
Back has two functions: go to previous "place" and cancel an in-progress control. (For more on the primary function, please read the excellent
Introducing the concept of Places by Peter Torr.)
The "un-expand" idea that Homeric noted really comes from the secondary function as a way of canceling an in-progress control. It's true that the ListPicker control accepts Back as a way of canceling its popped-up state, but it does that because Back also means "Cancel". That's why message boxes can be removed by pressing Back. (Note that tapping outside of a ListPicker's popup area also closes it and that pressing Back does not close an ExpanderView like the ones used in threaded email.)
If you accept the idea that Back means go to previous place or cancel an in-progress control display, that creates two problems. First, the Calendar behavior is wrong and second, the tile folder behavior is wrong. How so? Here's how:
For the Calendar, Back can't mean go to previous place because the screen never goes back to a previous place. While one could consider the Day Header to be an expandable control which Back cancels while it's in progress (i.e., waiting for you to select an event in that day), tapping outside of the control does NOT close it. This means that the Back behavior in the Calendar is inconsistent with the general OS.
For the Start screen, tile folders DO act like controls that are "in-progress" such that tapping outside of them causes them to close. (You can do this by placing one near the top or bottom and touching space not occupied by a tile -- the expanded tile folder with auto-close.) However, even though they act that way for touching, they don't from a Back button perspective.
Given all of that, I'm really pleased to see that Microsoft finally fixed an inconsistency that has driven me crazy since 7.0. Pre-8.1U1, tapping outside of the app bar when in the mail app did nothing; post 8.1U1, it now closes the app bar like it does in every other app.
Maybe there's hope for the company to address these consistency issues, too!
Sent from my Surface Pro 3 using Tapatalk