A more likely explanation is that at the time of the Buddha the duty of memorizing and reciting the texts was considered the province of the bhikkhus and bhikkhunīs. Although some lay people memorized discourses (Mv.III.5.9), and bhikkhus of course taught the Dhamma to lay people, there was apparently the feeling that to teach non-ordainees to become skilled reciters of the texts was not good for the relationship between bhikkhus and the unordained. There are three possible reasons for this:
1) People may have felt that the bhikkhus were shirking their responsibilities by trying to pass their duty off onto others.
2) Brahmans at the time were very strict in not allowing anyone outside their caste to memorize the Vedas, and their example may have led lay people to feel disrespect for bhikkhus who were not equally protective of their own tradition.
3) A bhikkhu acting as a tutor for a lay person wishing to memorize the Dhamma might, over time, come to be seen as the lay person's hireling.
It is important to know the context and in doing so, the rules all have a logical basis for existing.