"Lighting Model" is a property of the Default DAZ Shader. It changes how specular highlights interact with the diffuse colour, and turns SSS on if set to Skin. It has nothing to do with the ambient light effect. You don't see it before applying the Poser materials as the figures load with other, more advanced, shaders applied - the default shader is, as it's name implies, the default used when no other shader is set so it gets used on imports such as Poser files.

The "ambient" you are seeing is probably the preview's attempt to show the material - the important question is does it remain when you render? All your screenshots are in preview mode.