To give someone "free reign" means to allow them to do as they please; to not control them. The word, reign, refers to the leather straps used to control domesticated animals such as horses.
If you give a horse free rein, you let it go wherever it wants without trying to control it. The idiom uses the same definition, only with people - you've given someone permission to do something however they wish to do it, without trying to control things.
what are free rein leaders
Rein in these questionable questions
The cast of Free Rein - 2011 includes: Bryan Brown Rachel Ward
I am certain that it is free-rein leadership.Free-rein leadership - confirmed
He adjusted the horse's rein. Long may she reign.
What are some pros and cons of free leadership?
The king reined over the country. The rider took rein over his wild horse.
Rein, leash or tether
Leo X
In western riding it's called "The Regulator"
By giving free rein to diverse opinions.