I discussed this three or four years ago with a guy I knew on the web pretty well. He was a small town police chief in NJ. He said the reason for this is a "white paper" that was written in California specifying what police cars should have for lighting on them. In other words, someone who hasn't a clue, wrote this long paper on "proper" lighting, and now, for liability reasons, the industry feels that they need to comply with it.
Funny part is, when you come up on an accident or whatever at night and are faced with a sea of flashing lights, and blinded, the cops seem to act like you are a rubbernecker or gawker because you are driving so slow, when the fact is, you cannot see that guy directing traffic (he forgot to get his reflective gloves and vest out of the trunk of the crusier) and you are trying to not hit him. That happened to me a couple of months ago. He was frantically waving for me to get by (in the other side of a two lane highway) and I could not see him waving, or anything else till I was right on him.