Boredom is mind damaging too. Drugs could be alleviating a prisoners mental suffering by allowing a mental escape. And preserving their mental health in the long run. Something worth preserving assuming the person will reintegrate to society. But that probably depends on the person's propensity to get addicted.

If they will allow prisoners to MAID themselves, why is it worse to let them use drugs to reduce their suffering?

The bigger issue, IMHO (more damaging to society) is government corruption (looting, no bid contracts, bribes, over budget, under delivery services, and just comically bad ideas being funded). Billions of dollars have been misappropriated. And it's all legal. Or near legal, as white collar crime is rarely punished. And it can eventually turn into inflation and a declining standard of living.