I think the "rule" is about the practice of tuning one's bike and adjusting derailleurs upside-down.

I've seen people park their bike without a lock upside-down at stores, apparently thinking that a thief wouldn't be smart enough to turn it right-side-up, and ride off.

Yesterday I saw an unlocked Magna with the front wheel turned 180 degrees, again assuming a thief wouldn't be capable of turning the bars forward and riding.