Creativity allows a certain amount of free reign on use of words, although sometimes we tend to push a little bit too far and lose our meaning. “Reign” versus the more familiar “rein” in the previous sentence is one example of stretching meaning, but I hope still within comprehensible limits.
Free rein, according to a dictionary definition, means “to allow to act without restraint.” While there is no official assignment to free reign, reign meaning “to hold sway, prevail or predominate” adjectived (I love verbizing adjectives, then getting in even deeper by doing it to nouns as well!) by free, would seem to mean “at liberty to prevail.” Close enough in meaning, but with that slight twist that reveals the writer’s point of view and style.
I’d rather be a king than a horse, wouldn’t you?
The Lost Children: A Charity Anthology