We interpret the lines as lying on the image plane, so anything that obscures the lines is taken to be "in front" of the image plane. This trick works because

We interpret the lines as lying on the image plane, so anything that obscures the lines is taken to be "in front" of the image plane. This trick works because of the common human experience of seeing an image through various kinds of obstructions -- bars in a window, hanging strings, trees in a forest, and other similar obstructions. If we never learned to assemble an image lying behind vertical lines, the trick would not work.