It sounds like you figured it out, but yes...there isn't an 'off' setting for AF on the shutter release button, you just set it to something that doesn't include AF. For example, metering and AF lock.
Doesn't the 7D have an "AF-on" button?
Excuse my total ignorance but why is the BBF a good thing to practise ??
Back-Button Focus was added to cameras mostly as a result of heavy demand from sports photographers.
Here's the scenario:
Keep in mind that without BBF, your ability to shoot is on the _same_ button used to take an exposure reading and auto-focus. As a sports photographer, you'd put the camera into a "continuous" focus mode because your athlete is on the move and likely getting either nearer or farther. So you flip it into the appropriate mode (Nikon AF-C / Canon AI-Servo).
Now you're watching through the lens and tracking your athlete as they run down the field... and you're waiting for that "decisive moment" (often when an athlete does some key interaction with the ball such as throwing, catch, kicking, blocking, all depending on the sport.)
And as you do this... you're JUST about to shoot... when SUDDENLY, a referee enters the frame. And they're much closer to the camera than your athlete so naturally the camera gravitates to them as the new "focus target". You snap the shot and have a beautiful shot of a game ref... with a blurry player in the background. You think of lots of colorful metaphors which aren't polite to type in this forum to describe your opinion of what just happened to you.
WITH BBF:
Same scenario... only this time the _only_ thing the front shutter button does... is take the shot. It doesn't focus. You get to use your thumb to focus the camera but as it's completely independent, you can hold the BBF while your athlete is moving. As soon as you see a 'hint' of that referees body enter the frame, you take your thumb off the BBF so that the camera _remains_ focused on your athlete... if the athlete keeps moving past the ref, then you can put your thumb back on the BBF to let it resume focus. But even if you had shot when your thumb was not on the BBF, the focus would have remained at the last focus distance and the athlete probably would not have been able to move out of your depth of field -- so you still get a focused shot.
The BBF allows you to control when the focus should or should not change because you can choose when to engage or disengage the focus system independent of the shutter.
Recent and high-end cameras are even more advanced because they have focus tracking to follow action and it may be tunable focus tracking. Canon has a 47 page guide on how to tune the focus system on the 1D-X and 5D III cameras for any given sport or situation -- it's pretty powerful. With such a system, you can tune the camera to ignore periodic distractions passing through the scene... or tune it to rapidly jump to new subjects, it's all up to you.
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