Duh
Well that's just dim. Dim on the part of the people in charge, unless they did assess the risk properly and knew he really *couldn't* access any personal data, concluding that he couldn't do that much damage.
However, it's even more stupid on the part of the crim - he could have either a. Done the job as requested and used it to make himself a bit more employable on his release, along with probably time off for good behaviour, or b. done the job, left himself with a way in but not locked anyone else out, or c. deliberately 'failed', but still left himself with an admin account or backdoor, or d. done the dumbest thing of all and brought the whole situation crashing down around him. Sounds like a real evil genius, this one.