first I would like to say,I am not an expert, that would be Dan, and I am sure he will correct me if I am wrong, but here is what I believe is the problem
the 1157 bulb is only one part of the tail light,
the bulb,the reflector that sits behind the bulb and the lens are the others
and they must all work together in order to have a proper safe tail light
the bulb must be placed in the correct position within the reflector in order to allow the reflector to do it's job of reflecting the light from the bulb through the lens at the proper angles,
the lens and reflector are designed as a single unit, if you move the position of the bulb, or remove the reflector,
as with a flat LED panel, you compromise the lens's ability to work properly