I've seen this term applied to Bravely Default, Final Fantasy and other JRPGs, but I haven't been able to find a definition for it.
I have not found a clear answer to this, but I do believe it is a term used by the Japanese, referring to main characters always following their sense of righteousness - rather than giving the player a (moral) choice, which could affect the story.
In an interview with Akitoshi Kawazu, a director working for Square, the term is used to differentiate SaGa games from Final Fantasy; the latter being called the "rule of right" kind.
Wikipedia explains that SaGa games are known for their "open world exploration, non-linear branching plots, and occasionally unconventional gameplay." Final Fantasy games, however, are not known to branch out, and are linear in delivering the plot.
Another relevant, if less reliable, source is a post by Samantha Lienhard on the game Exist Archive: "it’s not a “typical rule of right” story. Moral ambiguity and shades of gray since the characters are all evil gods, I guess?"
Other reviews on Exist Archive appear to mention similar features, stating there are choices to be made which influence story elements.
Sources: http://www.rpgfan.com/features/creatorstalk/index1.html https://en.wikipedia.org/wiki/SaGa_(series) http://www.samanthalienhard.com/2015/07/first-look-at-exist-archive-the-other-side-of-the-sky.html