Why is lighting layer accessible when dynamic lighting is off?

...because it REALLY sucks when you drag your players onto a map and they instantly see EVERYTHING when you thought that drawing all those light-blocking lines was actually accomplishing something... Granted it was my mistake so the ultimate fault is mine, but I sure wish that making that mistake hadn't been possible in the first place. Would have kept a whole session from being ruined...

You might want to work on it while lighting is off to reduce any confusing shadows, light sources, vision cones, etc. I recommend setting the lines when the lighting is off, and only activating when you are ready to test. Also, there are other uses for the lighting layer, such as completely hiding objects so they are not even on the semi-transparent GM layer. You have my sympathies though. Revealing the whole map accidentally sucks. It happened to me recently (like you, this was my mistake-I had reset fog of war and forgotten to re-mask some areas).