I mean I cannot link you to a direct source, it was just something everyone knew, ya know 'common knowledge'. This was on both coasts as well as the midwest.
And it was explained to me at 3 different clinics in 3 different areas of the country that it was really about #1) liability -- particularly at clinics that accepted insurance for payment but not exclusively, there were cash-only ones with the same rule: No Benzos full-stop. If you had a legit prescription for xanax or ativan then they would send a letter to the prescribing doctor and would not dose you until they got an affirmative, positive response -- and to a somewhat lesser extent #2) they know it has the real potential to be fatal, and they're not monsters they don't want to kill all the junkies. Despite what you might think, some of them actually do give a shit and got into substance abuse medicine trying to help. Sure, for some it's just a job, and if you own the clinic it's a gold-shitting goose, but there are a lot of them who are genuinely trying to do good.