cats

The difference with all those other tools is that you are always there when it runs. You are not there when the robot lawnmower is running and some cat jumps your fence, which is why it should not harm the cat.

You are kind of correct that the rpm of the robot mower blades are faster, but the blades are much lighter and most mowers even have swiveling blades that are only held straight by the centrifugal force, so they will just swivel away when they hit something.

The blades are not much more than razor blades that are mounted on a plastic disc with one screw each. You wouldn't want to put your hand into the spinning blade, which is very hard to do anyway, but there is just not enough power to throw rocks with any significant force.

I would be able to put my finger in the way of the blades if they were completely blunt and I would not get hurt very much if at all, doing the same thing on a normal mower would be a very different story.