ACA mandates that ~80% of insurance co. revenue must go back towards medical service. So not "non-profit" per se but there is some kind of restriction there.
also 3) many "insurance" companies are in the provider game, meaning they can preferentially shuffle surplus to their other arm
(2) and (3) were part of what I meant by a lack of regulatory bandwidth in another comment. There are rules that could be enforced to promptly impose steep penalties for a company that tries to skirt them. But they just aren't, so after one company starts doing it the rest inevitably follow suit.
2) Not for plans that are (IIRC) two years old or newer. I'd be shocked if there aren't a bunch of shenanigans going on with this loophole.