My issue with mandated ranges is how to require the range to be narrow enough, without preventing the candidate from negotiating?

If you don't have any narrowness requirements, you can have a job where the range is $50k-500k, which is useless. If you narrow it to — let's say — 10% of the midpoint, you can say $180k-220k, but then you're not allowed to hire the perfect candidate who won't accept less than $250k, even though you would actually want to pay this much given that person's skills. If the candidate is allowed to negotiate upwards, ranges may gradually, over the course of years, drift downwards, with an implicit understanding that they are unrepresentative of the actual salary you are likely to earn.

With an infinite number of employers and an infinite number of candidates, you would always be able to find the perfect candidate for your mix of requirements and pay them the exact salary that matches those requirements. The real world isn't like that, though, and if you, as an employer, have an offer available, you will get candidates who are under and over qualified for the specific offer you had in mind, and you might nevertheless be able to find a spot for them at some salary that both of you are willing to accept.