In The Kite Runner, why does Baba insist that Amir receive religious training from the Mullah?

I think you are getting somewhat confused. When religious
training is mentioned in Chapter Three, it is not insisted upon by Baba, but a natural
part of Amir's education. In fact, as it becomes perfectly evident when Amir tells Baba
about what he is learning from the mullah, that Baba has little time for such teaching,
and counsels his son to be wary of what they say. He says, for example, when Amir brings
the topic up, that he has "confused what you're learning in school with actual
education" and then goes on to say that Amir will "never learn anything of value from
thos beared idiots." Lastly, having made his opinion perfectly clear about those
"self-righteous monkeys" he then says, ironically, "God help us all if Afghanistan ever
falls into their hands."

Thus we can see that Baba is
bitterly opposed to a God who has so little to worry about that all he can do is focus
on the way that Baba drinks and eats pork. He does his best to challenge the teaching
Amir receives and to give Amir a more realistic perspective on life, the world and
religion. I think Baba would have been happy if Amir did not have any religious training
at all, myself...