Why the material is not black when there is not Light???

Unity Engine
1

Here is my shader:

Shader "New Shader" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	SubShader {		
		CGPROGRAM
		#pragma surface surf SimpleLambert

      half4 LightingSimpleLambert (SurfaceOutput s, half3 lightDir, half atten) {
          return 0;
      }
		sampler2D _MainTex;
		struct Input {
			float2 uv_MainTex;
		};

		void surf (Input IN, inout SurfaceOutput o) {
			half4 c = tex2D (_MainTex, IN.uv_MainTex);
			o.Albedo = c.rgb;
			o.Alpha = c.a;
		}
		ENDCG
	} 	
}

When there is no light in the scene,I can still see the texture color.This is not as expect.You see,I already set the color to 0 in LightingSimpleLambert function.

How to make the material totally black when there is no light???

2

I’ve found it out,the Ambient Light in the Render Settings,God damn it!