Because it's an execellent metaphor is
referring to someone that is very fragile, they've had there fare share of bad luck before, so they know what it's like to be kicked when your on your very last legs. So they shouldn't throw stones (criticize) someone who's having a rough time, because they know what it feels like, and shouldn't what to do that to anyone else. Well that's the interpretation I get from it anyway. :)