Why do layered rendering not work with lights?

If I have a camera that is to render only one layer (say “Red”) and I have one item in the scene (in layer “Red”) and a light (not in “Red”, say “Lights”). I find that the camera will still render the light coming from that fixture even though it is not in the layer to render.

You need to set the culling mask of your light to the apropriate layers. By default it is set to “everything”.
It’s located at the bottom of the light inspector menu.