You buy it once, you are set for life. But for a human you have to keep on paying him as long as he works.

Yes but humans cannot malfunction, can they?
This is the first time these robots are being used in a real prison with real prisoners.
For all we know, they could have glitches that could provide the prisoners with a chance to escape.
Why run a risk like that when we could hire more staff for a more reasonable price?

Pfhahahahahaha

You realize that things usually get tested before they're used, right? Moreover, I think we established that the robots are NOT REPLACING THE HUMAN GUARDS.

HUMAN CANOT MISTAK.

I was also going to bring up the stanford prison experiment, but I think hanzo has done a fine job so far. Robots are not prone to the same sense of power corruption that humans are when they are put in a dominant position over many people that they didn't earn. Stating "humans cannot malfunction" is, as already pointed out, probably the stupidest statement you could make.. the amount of correct decisions humans make is usually far outnumbered by our mistakes.
And if the robots do malfunction its not like it would start some massive robot war, im sure it would be easy enough to shut off the system and start over if anything strange started happening.