It is talked about, but everyone I know who is in search and rescue is against it. Basically because some people will then be afraid to call for help and will needlessly die or suffer because of it. There are programs in several states that you can buy into for a minimal cost to offset S&R costs. In Colorado, you can by the SAR Card. $3 for a year or $12 for five years. If you are involved in a S&R situation, by buying the card you are covered. The S&R Group(s) who come to your rescue will be reimbursed through a general fund. This card is also part of Hunting and Fishing License fees, so those people are covered too. The other benefit is, if you buy one, you don't have to carry it. You're registered in a database that can verify you paid for it.
That said, some idiots who were just plain reckless have been charged with costs because they didn't have the card and they were stupid.
I don't think this is necessarily the case with these guys, though I would have thought they would be able to get down under their own power.
I get that logic, but on the other hand SOMEONE has to pay for the search and rescue. Either its tax payers who were not involved in the risky behavior, or the person who made the choice to take the risk. I get that there is a theory some people wouldn't call because they dont want to pay, so some type of insurance like you pointed out would be great. If they waive the insurance, and waive the call for help in hopes not to pay yet still do something which is universally known as risky and suffer or die...well thats Darwinism or (for you religious guys its gods will) that they suffer or die.