I've read it a few times in novels (not high-brow ones obviously). A few of them self-published and containing many, many, other grammatical errors, but also in several American novels which have been published and I assume have been edited, and which have otherwise acceptable English. Generally the phrase contains 'over', so a character might be given free reign over a particular section of a business for instance, meaning to take charge of it and not have to answer to someone else. So it does seem to be a slightly different usage to 'free rein' which is more of a running wild or unfettered image.