Pre and post render layer scripts are called only during 3D rendering and are not called during render target rendering.
Render target (2D) rendering is a post-process to 3D rendering. The 3D rendering code loops over all render layers and renders, and calls pre and post render layer MEL scripts. Then, when render target rendering is performed, the render layers are looped over again.
Doing 2D rendering as a post-process of 3D rendering ensures that all 3D render layer results are available for compositing, and that all 3D renders from render layers are done, regardless of whether they are connected to render targets or not.
For this second pass, pre and post render layer MEL scripts are not called.
