> In the end employers are the ones largely paying and they are professional negotiators enough to put price pressure on insurance plans. 20% of $0 is $0.

That's assuming price is the only variable.

Suppose one insurance company is accepted by more providers, including ones that might be closer (but pay higher real estate costs) or have nicer rooms etc. Meanwhile employers are looking for cost/benefit rather than just cost. If they give employees a better insurance plan they could pay them less or provide less of some other benefit and still get people to work there.

So before the insurance company didn't really care if you got a $10,000 plan or a $20,000 plan if they both had a $2200 margin, or if anything would prefer the former because they make the same money with lower costs. The employer is likewise fairly ambivalent as long as the more expensive plan seems like it's buying something (even if the something is convenience/luxury). But now the insurance company isn't allowed to have a $2200 margin on the first plan and still is on the second, so that's what they market, and then what more employers choose, resulting in higher average costs.

> Insurance providers also rarely operate at the full freight 20% either way though.

There are only really two options, right? Either the market is actually competitive and then a margin cap has no effect because competition would prevent margins higher than that regardless and the rule should be gotten rid of as totally redundant, or the market is less than perfectly competitive and then it does something but the something is a bad perverse incentive to raise costs to cheat the rule and it should be gotten rid of as actively harmful.