CrackBerry Genius of Geniuses
It's done this way I think because you've already interrupted your original work, so it wouldn't always be desirable for the back-swipe to send you back to the same app.
The original gesture is carefully "reversible", following the principle that if the user doesn't release the gesture to complete it, "retracing" the gesture path back to the start undoes whatever it started.
If you swipe up a bit, the running app minimizes part-way. You can see icons at the left immediately, with "splats" and numbers representing new messages or notifications to be dealt with, and the icons representing the different types of notification (e.g. phone calls, emails, twitter msgs, etc). If you don't see anything important enough to interrupt your work, you just return your finger to the bottom and are back in the app.
If you swipe up to that point, and see something you think might be relevant, such as a new email, you continue the gesture by moving to the right, which incrementally reveals the full Hub with the important parts of the notifications shown. You can quickly tell if that last notification was a message, who it was from, and what the subject was, or for a phone call you can see the number of the caller and when it arrived (or things like that... I'm speaking from memory). Again, the gesture is reversible so if you don't want to go and view that new message in all its full glory, just move your finger back down and you're right back in the original app again. The feel of this is all intended to help you maintain your "flow" in whatever task you were doing, since there aren't sharp transitions and no "modal" effects going on here.
If, however, you choose to complete the gesture, and actually go into the Hub's full view, then it's taken as a sign that you've now interrupted your task and are focused on the new message(s). You spend only a few seconds here, but quite possibly you could be taking a real break from the task or not even planning to return to it (e.g. if it was a game that you didn't care to complete).
Since you've interrupted your work by completing that original gesture, you return to it by doing the right-swipe to get to the active frame page, and if you really want to go to the original task it's always right there in the upper-left position waiting for you, with a single extra tap as you noted. Since you did interrupt your work, however, odds are pretty high that when you go back to that active frame page, you'll be ready to check on something else that was running, or even continue past it to the icon pages and run something new. You may still not go back to that app very quickly.
I suspect extensive usability testing has shown that although sometimes this obviously won't be the thing you want, more often than not it is, and the effort to do that single extra tap is so minor that it's worth the tradeoff. The alternative would be to make "return to previous task" the one-swipe action, in which case you'd probably need to take more steps to do the alternative.
I really doubt you'll find this a serious problem in real use, because you'll learn to use that original gesture's reversibility to full effect, and will only complete the gesture and go to the hub when you really do want to interrupt the task, at which point returning much later via the swipe-tap combination will not feel like a problem.