It's been discussed a bit on the forums.
The icons +/- is because we don't have depth perception, which we would in the real deal. The range is because you can use depth perception to have a good idea how close or far something is, but on a 2D pixel-based screen you can't. The icons in general are because you can see a lot more detail at long range than a single black dot of a pixel can tell you.
Note, however, that if you fly low enough your icon displays UNDER your plane, making you more stealthy. It disappears below the ground and can't be seen. I think it's a couple hundred feet or less, but I don't know for sure.