Layer "lock", a weird situation, a bug, or ?

Ok, I was talking with someone who was having a rendering problem. basicly, a normal render (f12) was working perfectly, but when he was trying to render an Anim, the output was completly black.

Seeing that by pressing the Anim button still made the computer work a bit (render was not instant), I understood his Light layer was not taken into account on rendering. I started looking for potential problem and saw that the “lock” button on the right of the layer buttons was desactivated (not lock). I clicked to lock it and rendered again making sure both layer were active, and it all worked fine this time around.

Now, I thought this button was made to let the user have different layer activated in different 3d views.

I’d like to know how it really works, and if it’s only purpose is what I thought (having different layer in different 3dviews), then is that a bug?

thanks in advance
here’s the file
http://h4x.snipanet.com/blender/mirror/test.blend