Insuring the Youth

Most of the people who doggedly follow health care reform know that one provision of the ACA forced insurance companies to allow young adults up to 26 to stay on their parents' insurance.  I find aspects of this bizarre.

First, it's probably the case that these people will have low health care costs.  Therefore, if they were participating in a perfect insurance market, the cost of insurance probably would have been low.  Why would it be in their interest to move to the parent's plan. Presumably, the cost of adding them to the parent's plan would be higher than the cost of individual care.  Therefore, they'd be subsidizing others.  Again, why is that in their interest?

Possible answers: parents would be paying, so it's cheaper for the children. But then, the parents could have paid anyway.  Still, I think that's the most likely explanation, as their are psychological reasons that'll push parents to pay when they're actually receiving the bill.

Secondly, why didn't the insurance companies do this in the first place?  If they weren't doing it before, that implies it wasn't in their interest.  But many companies have announced they'll maintain this policy even if the ACA is struck down.  What changed?  My best guess is that the insurance companies would have done this eventually, but the process was accelerated by the ACA.

Also, in this article at thehill.com, the author writes "Adding more young people to the insurance pool is popular in part because it helps lower premiums for everyone."  I think they meant to say "everyone else."  It costs more for the young people, of course.