why does my temp camera only renders transparent objects ?

Sometimes in game I have to get pixel data from the screen, to do this, I render the scene again to a small rendertexture, from which I then read the pixels (I would be better and easier if I could just get the render from previous frame, but ah well)

the code is this (in a script attached to the main camera):

 var tex:RenderTexture = RenderTexture.GetTemporary(400,400);
 
 RenderTexture.active = tex;    
 var thiscam:Camera = GetComponent(Camera);
 var cam:Camera = (new GameObject( "tempCamera", Camera )).GetComponent(Camera);
 cam.CopyFrom(thiscam);
 cam.targetTexture = tex;
 cam.Render();
 
 var tex2d:Texture2D = new Texture2D(400, 400, TextureFormat.RGB24, false);
 tex2d.ReadPixels(new Rect(0,0,400,400), 0, 0);    
 
 var pngShot:byte[] = tex2d.EncodeToPNG();
 File.WriteAllBytes(Application.dataPath + "/testColor.png", pngShot);
 
 RenderTexture.active = null;
 cam.targetTexture = null;
 DestroyImmediate(cam);
 
 RenderTexture.ReleaseTemporary(tex);
 
 return tex2d.GetPixel(200,200);

for testing purposes, I made the texture bigger and save it on desk, and so I noticed that only objects with transparent shaders are rendered.

Any idea why this is ?