I have a "LAME " question?

okay this has had me thinking all day... In the old cowboy movies a horse would come up "lame" and im thinking it was always the bad guy would shoot his horse ..so my question is .. WHY ?? .. isn't it just a stone or a pebble that got caught in the hoof and then wouldn't simply removing the stone and letting the horses hoof regrow eventually and then the horse be fine ? so why then destroy a horse for a temporary problem ? anybody know the reason behind this ?