That light pollution has only existed for a tiny tiny fraction of the time since life first evolved on Earth. It's entirely conceivable that in another 1000 years (a blink of the astronomical eye) it will no longer occur - so trying to detect whether life exists on any random planet/moon that way seems a low probability exercise, but granted if it's cheap to do you might as well (conceivably there might be other detection methods based on electromagnetic radiation etc. that would reveal more information on the dark side of a planetary body too).
