It's how Ambilight works. I have 7304, and 7101 - on both when is black picture, then LEDs dimms but not off completly. I have tried to display 100% black picture and the same situation. In the past I have contact with Philips and they confirm that this behaviour is correct.
Philips has a Tutorial Video (in German) where they say that the Ambilight shuts off completely when black content is displayed... So I assume this was just a lie to make people buy their stuff... Very sad. Last Phillips product for me!
I like 'Natural', precisely because it switches off in the darker areas. If the idea is to make your screen feel slightly larger, then it's really the best option.
Worth also flagging up another option that's a little bit buried but useful for some people - setting the wall colour. My TV is mounted on a dark blue wall and so ensuring the Amblight knows that means it adjusts the colours to account for the different colour surface. It's by no means perfect and when a white light is called for, there's obviously nothing it can do, but every little helps.