Normals are essentially the direction that a face is pointing, as you say, but your polygons do not have normals on the inside and outside. The reason the faces are shaded on both sides is just Maya's best effort. You still may have problems with the side opposite the normal facing direction. An easy way to see this is to turn off two sided lighting (in the panel menu > shading > two-sided lighting) The "wrong" side will now be black. Hope this helps.