It's worse than that. The ACA limits the percentage of their premium that can be used for all non-medical costs - including the insurers' profits, yes, but also including administrative costs like paying people to investigate potentially fraudulent claims. If they put resources into (say) catching that one guy who was fradulently billing them for workout advice at inflated rates[1], not only does it shrink the pool of money they're allowed to keep as profit, the cost of the investigation comes out of that pool and eats directly into their profits. The Affordable Care Act effectively penalizes insurers for rooting out false or inflated bills.
[1] https://www.propublica.org/article/health-insurers-make-it-e...