Thumb cropping is an ancient method of enforcing discipline or punishing an enemy. Almost all ancient cultures employed it at one time or another because it made it impossibe for a foe to wield a weapon.
Just be thankful that the Mongols aren't running Rendo, or they'd be boiling you in oil after lopping off your naughty bits. 
Say....hold on a sec. 
This whole rhubarb started over the new policy forcing images to be cropped of any potentially offending naughty topography. What's that I hear? The twang of bowstrings and the pounding of horse's hooves?
Sorry. It's just my wife yelling at me again.