I noticed this with BHX and MAN the other day. Though interesting LHR is full of planes.
Reading a bit more about it, the suggestion is that its not for the purpose of security - after all what would knowing which planes use an airport tell us. Its more to do with the fact that google maps, or any such tool, is meant to be a capture of the land, not the land and cars and trains and planes and people etc. Evidently, clever algorithms have been used to show the land as it should be, rather than with a plane on top of it. Which makes some sense.
I assume it falls off the fact that every bit of land will have multiple shots of it. Thinking of an airport, the system probably crops together multiple images, e.g. one showing gate x empty, the next showing gate y empty and so on. The point being that each image taken will be different if there is a movable object on it. This is probably most notable on train lines - there are very few trains on google maps!