Heroku Buy AWS services at standard AWS Price (X) * (Substantial) Discount.

HeroKu Sell their services at (Y) where currently Y is >(X).

If they had acquired Heroku by Selling it at AWS Price (X), Amazon is still earning where they previously were not earning from those heavy Discount. At the expense of the Operational Cost of Heroku Team. And the benefits of further lock in and additional market segment which is currently not very well served by AWS.