They are virtually all that way. It makes it a lot easier to see the radar screen over an extended period of time, especially with older CRT's.
There is some history to this. Displays used to update in sync with the radar in real time. The images were merely displayed, not stored in a computer as they are today. Images on CRT's would fade away gradually, so the air traffic controller could see them for at least a few seconds. Before the sweep images faded away completely, they would refresh (on the next sweep). Because the images grew dim as they faded, it was necessary to make radar rooms dark, to aid in viewing.
Today the displays are much higher contrast, at least in most facilities. But the practice persists, probably due to an FAA human factors study.
Dave
