Lawnmower robots have much smaller blades with less inertia. I don't doubt a direct hit would hurt, but rocks just won't get thrown very fast by it. A direct hit would also be pretty hard to achieve because the skirt stops anything big from getting under the robot and the blades stop immediately when it gets lifted up.
Just imagine how many lawsuits the manufacturers would have to deal with if these things were only slightly unsafe.
Yes, I have heard about that too, which is why I only let mine run at daytime.