If one is holding U.S. passport then most of the time information regarding return journey is not asked. However, the rule does exist, temporary visitors to Japan do need to produce outgoing flight of ship vessel information indicating the date of departure from Japan.

From my colleagues at work, I know people holding passport from some countries (I will not going to bother to list here) are almost certain that will be asked about outgoing flight/ship vessel from Japan.

Also, a person have to be from a country where Japan has visa exemption for short term stay to do stuff like this. The answer may not be easy as “yes, you can do it no problem.” Cannot always assume that people are from same country as you are.