In this story, Aram and Mourad have their own unique definition of stealing. For a start, their definition of stealing only relates to high-value commodities, such as money. Anything of less value, like a horse, cannot be considered an act of theft.
Secondly, Aram and Mourad believe that stealing involves selling the stolen item to somebody else. In other words, if they decided to sell the horse to somebody else, that would make it an act of stealing—not the act of taking the horse.
As a result of their unique definition, the boys do not believe that taking John Byro's horse constitutes an act of stealing. After all, they are "crazy" about horses and fully intend to return the horse to Byro after they have learned to ride. So, in their opinion, they have simply borrowed the horse and cannot be portrayed as criminals.