The legality has nothing to do with it, as it has been demonstrated over and over that law enforcement will employ methods that can charitably be described as "legally questionable" and then attempt to hide it (see stingray). What is stopping them is limitations imposed by the physical world - both in number of windows and the fact that laser mics don't work the way they're portrayed on television. This is the reason why digital key escrow is infinitely more dangerous than physical key escrow (like fire department lockboxes).