I would have expected them to prefer a prepaid plan, they get all the money up front. Isn't that why they give you a discount, to encourage it? But I guess the discount is why they prefer a postpaid plan? Very odd, I wonder why the prepaid plans even exist, or are discounted.
Postpaid plans (the traditional ones which have contracts) usually give discounted or free phone upgrades while prepaid does not, so they can bring the price down.