I've had many of those forehead slapping moments myself over the years.
Here's the entry from the After Effects Reference:
"If transformations are not collapsed, a nested composition that contains 3D layers is rendered as a 2D image of the 3D arrangement, using the default composition camera. This rendering prevents the nested composition from intersecting with 3D layers, casting shadows on 3D layers, and receiving shadows from 3D layers in the containing composition. The nested composition is also not controlled by the cameras and lights of the containing composition."