Though the story definitely teaches that there is no excuse, not even poverty, for taking something that is not yours, the bigger moral of "The Summer of the Beautiful Horse" is that if one understands another's circumstances and motivations, wrongs can be righted without anger or retribution.
Some boys take a horse that is not theirs to take. The owner catches them but rather than accuse and demand retribution, he shows understanding even as the boys do not at first admit to the wrong doing. However, later the boys do show understanding that this was wrong and make a sincere effort to return the horse to the owner. In returning it, the owner takes into consideration and understands the boys' circumstances and motivation. Without anger or demands for punishment, he is able to see the situation clearly and realizes that the horse has actually gained benefit from the situation. He is more behaved and manageable because of the boy's interactions with him.