Why don't light emitters reflect light? I'm testing out an edge case in Cycles.

Here's the setup: I have two area lights: a large dim one and a small bright one. There's also a ground plane. Each light has a box behind it prevent light from shining from the back side. The large dim light emits a small amount of light downward and the small bright one emits light upward. The small one is positioned between the large one and the ground plane. Here's a diagram of the setup:



When I render the scene, I would expect that light from the small light should reflect off the large light and onto the ground plane below. However, that's not the case here. When I change the brightness of the small light, the illumination of the ground plane doesn't change at all. That suggests that there is no light being reflected off the large light source. Here are some example renders:



When I hide the large light, the light from the small one bounces off the object that was behind the large light and, as expected, illuminates the ground plane below. Some examples:



So my question is: why don't Emission BSDFs reflect light? In theory, they should; even the Sun reflects light (albeit a minuscule amount compared to what it emits). Is this a design choice or a bug? If it's a design choice, then why?
This is expected behavior for Cycles and in line with the way a physically based render engine is supposed to work.

By assigning a purely emission shader to a surface you are telling the render engine that that surface has no other light path component interaction, and is only emitting photons, no light is absorbed, no light is reflected, no light is refracted.

If you expect more light path behaviors from said surface you have to add the corresponding shaders for those.

What this means is practically you have to use a Mix shader to mix the emission with other materials cabale of reflecting light from other sources, in this case a simple Diffuse or Glossy shader will do.

For clarity sake the "strong light" is red colored and the weaker one is blue in my scene.



Also as mentioned by Cegaton, you don't specifically need those boxes, if your only intent is to stop light. You can either use actual Lamp objects set to Area which will by default only emit light in one direction, or use a node setup similar to the one below to limit a certain shader to the front facing geometry only.