It all depends on the object. These glass models have thickness modeled into them, so putting the camera where the liquid would be it's still on the "outside" of the glass. With polygon models the renderer has to guess what's "inside" and "outside" by looking at the normals. If you just have a plane and you move the camera to the other side of the plane, then you're on the "inside". But the Glass Shader has a checkbox for that situation so it can treat it as though it's still on the outside.
Matt