...is funny when one prisoner is telling it to another. It's commiseration, a way to cope with shared hardship. When the executioner is the one telling it, it's just cruelty. This is why jokes about some terrible things, like the general unfairness of the world, work: because we are all in Schopenhauer's words fellow-sufferers. Nuclear war, death, and aging all fall into the same category. When you belong to the group that is statistically far more likely to rape - not to mention the group that benefits from rape culture - telling rape jokes is not funny.
Look at it this way. Police brutality jokes work best when they're being told by members of the groups that suffer the most abuse (say, Dave Chappelle), but they work pretty well in general because most of us exist below the cops in the hierarchy of everyday power. If, on the other hand, a policeman is telling police brutality jokes...
At best it's grossly insensitive. At worst, it's a threat.
(h/t to the Pharyngula commentariat)