Insurers have to compete against other insurers on price. Otherwise they lose market share. All of them negotiate hard with providers to drive down prices.
But total medical spending growth slowed, which seems to indicate that this price inflation didn't happen.
That's not a cost control. That's a price inflater!
If the only way for them to make more money is to have the price of things go up, what do you expect they're going to be in favor of? In a perfect world (from the perspective of the insurance company), everything from premiums to procedures goes up X% so that their profits go up X%.
That's a seriously messed up system and may be the most egregious part of the entire ACA.