Sainthood is not determined by the pope; sainthood is only officially recognized by the pope through the canonization process of the Catholic Church. In other words, all people who die in God’s friendship become saints (after purgatory, if needed). In some cases, the Catholic Church officially recognizes the sainthood of certain individuals by canonization. It should be noted, too, that canonization is a later development in Church history; originally, saints were recognized by the acclamation of the communities in which they lived and by their bishops.
Basically, Eastern Orthodoxy can have saints because Eastern Orthodox may die in God’s friendship. In the Catholic view, the recognition of certain Eastern Orthodox to be saints by Eastern Orthodox churches may be somewhat similar to the acclamation process of the early Church and the bishops. But the sainthood of these individuals is not presently recognized in the Catholic Church, though their salvation is hoped for.