The point with the cable is that extension cables aren't allowed by the USB standard, so Apple created a cable that would only work with their specific device for which they guarantee that it works. I've tried the extension cable with other devices (it works just fine if you insert the plug at an angle), and have had mixed results.

I think Apple did exactly the right thing here and did it to avoid other problems ("I plugged in my iPad through the extension cable to my macbook and now it won't charge").