Traitors tend to not id bodies because if they do, that will narrow down the alive people and make it more obvious that the last group alive is mostly traitors. Traitors also do not id bodies because they think that they aren't being watched, and the body may reveal their location. I do agree on dark maps like clue that it is hard to id bodies when the lights are off, but you need to remember, you have a flashlight! XD
If kos for not id'ing was removed, it would be an insane plus for the traitors, to the point where people may even not id bodies anymore. This would disrupt the balance of the game between traitor and detective/inno.