Credit card companies are the largest buyer of miles from the big US airlines. By extension they’re the largest customer for air travel, too. They’re paying billions of dollars and they care what they’re getting in return because they need those miles to be valued by cardmembers in order to incentivize spending on their products.
So why don’t card companies raise a stink when airlines devalue their miles? After all the card companies are getting something worth less for their money. Banks aren’t usually known for being pushovers as counterparties.