I dont really know much about tin foil, I just have always been told and everytime i have heard of someoen using it to stop and not to.

But I woudl asume since when it gets wrinkled, creases, bent, or whatever it would magnify or intensify the light, like a magnify glass. Just like the same idea not to use glossy paint. If you look at glossy paint with a light on it you can see where the light is hitting and in that spot its even brighter.

I dunno if that makes sense or is even right, jsut my thoughts on why not to use it. Hope it helps