EDIT

Logic inference has kicked in:

The reason it is recommended that the chain should be mounted one way is that the curved/shaped lower part (as opposed to the flat upper part) allows its use on very low tooth count sprockets (down to 9T) - as the curved/shaped lower part of the chain allows it to bend/wrap around small diameters.

This does not mean you cannot mount the chain backwards - just not upside down.

On larger rear sprockets even this consideration can be ignored.

Hope all that makes sense !