Serving food is simpler since you can just carry a cart with say a soup pot on one entrance and hand in bowls of it while the thing rotates. With moving door you'd have to keep moving the pot or pre-laddle the soup.

Same goes for all other housekeeping tasks. When the prisoners take their linen and clothes to wash they can not have any contact with other prisoners as they don't walk past their cells.

Bringing prisoners in is easier since they will always move along the same exact route no matter what since that is where the door is.

You could also potentially keep the prisoners from knowing who is in the same jail as them if you don't let them see one another. Obviously sound still carries, but some prisoners might not want to talk, so you could also consider it a privacy feature.

It also means you can firmly attach the bars to floor and ceiling making the cells more secure.

These are just off the top of my head, I'm sure there are others as well. While I can not think a single good reason why the bars should move.

Alternatively, they could simply have a external lock on rotation that's engaged when someone's not actively moving the cells.

1: https://en.wikipedia.org/wiki/Worm_drive

Only real fire related threat I can see is if a prisoner for some reason wants to set himself on fire and succeeds. Putting him out might take too long.

As for the inhumane part. I personally don't see it and I think our prisons are little too nice currently, but I'm from a country that doesn't run for-profit prisons.