It doesn't require collusion, just misaligned incentives.

"Perversely, with the MLR requirement capping profit margins and administrative costs, insurers are discouraged from containing health plans’ premium increases. Economists have noted that the MLR requirement effectively turns health insurers into “cost-plus” businesses: If insurers’ predicted premiums are less than the actual medical care spending on claims, it can lead to higher MLRs and less profits, within MLR restrictions. Professor Scott Harrington warned early on that MLR requirements could reduce insurers’ motivation to control premium increases. Prior research has found that the MLR requirement is associated with stronger financial performance for insurers, since they can raise premiums to cover higher claims and still comply with the MLR threshold."

That’s an opinion from an economist along with prior behavior, not reality. I’ve talked to one of the guys who worked on the ACA, and his take was basically providers will do things like e.g. buy up all the cardiologists in an area, then jack up prices across the board. Employers need to provide a certain bundle of services under the ACA. That’s where people are taking advantage.