The tails are docked so when they are driving so plowing reins and straps can't get caught under the horses tail. Its also done for management. Drafts have thicker more unruly hair than smaller and lighter breeds and fussing with a tail is not something a farmer wanted to deal with constantly. It's now a traditional look but the Amish still use it for safety reasons as previously stated.
♪♥♫ A horse which stops dead just before a jump and thus propels its rider into a graceful arc provides a splendid excuse for general merriment.
~Duke of Endinburgh ♪♥♫