To understand this question, take a look at Aram's thoughts and feelings when he is first offered the chance to ride the horse by his cousin, Mourad:
Well, it seemed to me stealing a horse for a ride was not the same thing as stealing something else, such as money. For all I knew, maybe it wasn't stealing at all. If you were crazy about horses the way my cousin Mourad and I were, it wasn't stealing.
In Aram's mind, when Mourad took this horse, he had no intention of selling it to make a profit and, therefore, his actions cannot be described as theft. As such, he does not need to justify what has happened because he does not believe that his cousin, Mourad, has broken the law.
Moreover, Aram is easily able to justify his own riding of the horse. He allows himself to ride because he is "crazy" about horses and this provides him with a rare opportunity to indulge his passion. His family is so poor that this chance might not come about again so he must seize it, before it is time to return the horse to its rightful owner.