Why the "target texture" option is enabled in Unity iPhone Advance for all the cameras?

This is hardly a special case. Every mesh has "Cast Shadows" and "Receive Shadows" checkboxes. You can use shader code that won't do anything (GrabPass and SeparateSpecular On for example). Look at the Quality menu for a whole bunch more examples of useless-in-Unity-iPhone aspects of Unity.

There is a limited ability to port projects between Unity and Unity iPhone. Some stuff just won't do anything, but won't generate an error, either. If all of this was taken out, then you'd have to set up all the Unity-specific options again, every time you used that Editor. (As it is, you can see an example of this annoying-ness: if you go from Unity to Unity iPhone, your iPhone-specific Project Settings are overwritten.) Consider this something like Fallback Shaders.