Magic Mouse on/off switch

Hi, I just bought a magic mouse and am loving it but I can't figure out if the on/off switch is faulty or not.

It doesn't seem to make any difference if the switch is on (green patch showing) or off - the status light still glows and I can still use the mouse. I'd assumed that in the off position the mouse would be... er, off?

I've searched everywhere for an answer to this and read the little instruction book that comes with the mouse but can't find a description of how the on/off switch should behave.

Sorry if its a dumb question but I'd like to turn it off when not in use and know that its not using up the batteries.