MEGAN MCARDLE: Aetna’s Retreat From Obamacare Is More Than It Seems.

The question is: What matters to regulators more? Blocking the mergers, or keeping the exchanges healthy? That’s not an easy question. As of this writing, it looks as if Aetna’s withdrawal will leave at least one county — Pinal, in Arizona — with no insurers at all selling exchange policies. And it seems unlikely that Pinal County will be the last to lose all its insurers unless something pretty drastic changes in these markets.

The state regulator has made hopeful noises about persuading someone to pick up the business. (Remember the regulatory goodwill we mentioned above?) But regulators in relatively small states don’t necessarily have that much clout with big insurers who can afford to keep taking these losses for years. California can plausibly say “Play ball with us or get ready to lose our nearly 40 million citizens as potential customers,” but a big corporation probably does not tremble in fear of the mighty market-shaking powers of the Vermont Department of Financial Regulation. And more locally concentrated firms cannot simply keep eating large losses for an indefinite period. It is obviously a problem — for politicians, as well as customers — if a growing number of people have a theoretical right to buy health insurance but cannot actually buy any.

People no longer have a “right” to buy health insurance; we have a legal obligation to do so. That’s going to be an increasingly difficult obligation to fulfill as insurers abandon one ObamaCare market after another.