This sort of thing is quite common:
Recruiters who continued to meet their quotas were given almost free reign, they say.
It should be ‘free rein’. The metaphor is horse riding, not monarchy. If you give a horse a free rein, you let it wander where it likes – the opposite of keeping a tight rein on it (see also ‘rein in’ and ‘hand over the reins’).
But ‘free reign’ also has a logic to it: to reign freely is, obviously, to have more power and independence than to reign with checks and balances. However, ‘free reign’ doesn’t have a record of being literally used to describe an unconstrained monarch. This logic is just rationalising a homophone.
In fact, the earliest uses recorded by the OED, from the 17th and 18th centuries, are in the plural form, ‘free reins’. This makes plenty of sense for a horse, while ‘free reigns’ would make very little sense for a monarch. The Google Books Ngram shows the singular form being roughly twice as common as the plural for much of the 19th century, then shooting ahead from about 1875.
‘Free reign’ popped up occasionally during the 19th century but it only really edged into noteworthiness around 1930. Since then, it’s grown more and more common:
In 1950, ‘free rein’ outnumbered ‘free reign’ 25 to 1; by 2000, its lead was just 5 to 1.
But ‘free reign’ is still generally regarded as nonstandard.
Robert Burchfield says that it is a mistake. Bryan Garner rates it a stage 2 usage (‘widely shunned’). And Merriam-Webster, despite its limp conclusion that “If you are not sure whether you want rein or reign, your dictionary will help you,” says that ‘free reign’ is the result of a confusion. It blames the decline of horse riding for the misunderstanding of rein-based expressions.
So it’s about horses, not monarchs. There’s a good mnemonic that can help here, thoughtfully provided by William Shakespeare:
A horse, a horse, my kingdom for a horse!