I think this is because libertarians don't believe in health insurance as a means to help people with health conditions pay their bills.
I would put it this way:
Insurance is a voluntary arrangement where consumers agree to subsidize each other. By definition, sick people have higher medical expenses. Thus, some seek to charge healthy people more than they cost to insure, so that insurers can reduce the premiums they charge to the sick.
There are lots of reasons why healthy people may agree to that. They may be very risk-averse, and so they are willing to pay more than they cost to insure. They may be altruistic, deriving satisfaction from knowing that their higher premiums are making coverage more affordable for others. Or they may precommit to such subsidies before it is known who in the insurance pool will develop a chronic illness (read: guaranteed renewable insurance).
As a libertarian, I have no problem with the healthy subsidizing the sick via private health insurance — so long as the arrangement is voluntary. But problems arise when public policy tries to get healthy consumers to provide, shall we say, "extra-voluntary" subsidies:
- The healthy people eventually figure out that they are being over-charged, and they bolt. That makes the risk pool less-healthy, premiums rise, and more healthy people leave. Lather, rinse, repeat, and you've got your very own adverse selection death spiral.
- The insurers realize they can't make money off the sick people, so they avoid diabetics and such as if they had the plague.
- And it doesn't help the situation that forced subsidies lead to greater moral hazard among the very people who already use lots of medical care. That just fuels the first two responses.
So to tweak Paduda's characterization, libertarians think private insurance is a wonderful vehicle for voluntary subsidies and a lousy vehicle for forced subsidies.
In a world without such forced subsidies, Paduda is correct that we would purchase a lot less health insurance. And I find this comment instructive:
[I]nsurance would not be available at any kind of affordable price for anyone who really needs it if Cannon's prescription becomes reality" [emphasis in original].
Sick people don't need insurance. Insurance doesn't make sick people healthy. They need medical care. They may even need subsidies. So why not try to provide them those things, rather than wreck the markets for both health insurance and health care?
Many equate insurance with subsidy. In fact, one is a subset of the other.