Radiosity normal map light baking
The lightmap representation used in Valve's stuff, like any finite-storage representation of the surfaces response to light, is only approximate, and the error in the approximation is higher or lower in different scenarios.

Since it doesn't store the light in a direction tangent to the surface (where it would be exactly zero), you don't get an exact zero for this case. However, this isn't that important:


Normal maps generated from height maps will never yield a normal vector exactly perpendicular to the geometry, and tend not to be not that steep anyway.
The 3 generated lightmaps are normalized so that their sum is equal to the light for the "straight up" direction. This reduces directional contrast, but also eliminates light from the tangent direction.