Why does the backlight turn on when the lid is closed?

In your displays you have "Automatically adjust brightness as ambient light changes" in "Displays" in "System Preferences" enabled. When this is enabled and sleep is disabled it will turn on the backlight.

Insomnia 3.0.1 on Snow Leopard and Lion (not tested on other versions yet) would enable the backlight temporarily but shut off afterwards. It does this by shutting down the AppleLMUController which detects ambient light sources.