It is also important to understand that the requirement of return or onward ticket for tourist entry is requirement of Japanese immigration. It is a common rule where the U.S. has the same rule. It is not just if an airline will allow to board the flight, when you arrive at Japan, passport control at the port of entry at Japan may deny you entry.

I read that a tourist traveling without specific plan/itinerary. Not have a plan where to visit, where to go in Japan. Not have a plan on how long staying in Japan and have no plan when to go back home. These tend to be a red flag at passport control. Once you denied entry to Japan, you will not be allowed to enter Japan for next five years. Even after five years there is good chance you may need to get tourist visa in advance of travel to Japan due to your past situation

When a person is denied entry to Japan, the airline which carried the passenger is get fined by the government. This is also common policy, not just Japan, but the U.S. and many European countries have such rule. This is the reason airlines check passport and other entry documents of passengers.