Or gardens. They have the time to tend a garden and feed chickens and collect eggs.

I'm guessing a third party has a fat contract to provide bad food.

And more effective. IIRC, while vitamin deficiencies have been shown to cause problems, vitamin supplements haven't been shown to "fix" that problem [0]. You can't eat food with no nutritional value and then add the vitamins back in...that doesn't work. You need to eat food that has the vitamins to begin with.

[0] https://www.hopkinsmedicine.org/health/healthy_aging/healthy...

>> At least it is the common sense medical advice: "you don't need vitamins, just a healthy diet".

You also need sun (Vitamin D), which is not always an option for prisoners kept inside for 24 hours per day.

I can imagine that from a bureaucratic perspective, buying vitamin pills that have an extremely long shelf life is attractive and gives the veneer of paying attention to the nutritional health of inmates.