Long tail costs are things like freak car accidents, hail, etc. Reckless drivers should and are charged more because their costs are higher as a result of their own choices. Obese smokers should be charged more than healthy people but they're not because ACA says you can't discriminate based on pre-existing conditions. That means healthy people end up subsidizing lung cancer treatment, heart disease treatment, insulin, etc. for fat people.
edit: correction, ACA permits charging tobacco users extra. But not, for example, weed users.
Yes, because age (like smoking) is one of the few factors ACA Marketplace plans can take into consideration. But healthy older people pay the same premiums as unhealthy older people, which means one of these two groups is being overcharged. Moreover, it means that there is less incentive (not none, but less) to make an effort to end up in the first group as it won't affect your health insurance costs.
> The same way responsible drivers should not pay for reckless ones? Or responsible homeowners shouldn't pay for reckless ones?
Yes, exactly. Reckless drivers and reckless homeowners should pay higher premiums, as their insurance covers a higher risk (a priori expected cost) compared to responsible drivers and responsible homeowners.
> The whole idea of insurance is to distribute the costs so that no one has to bear long tail costs alone...
The "whole idea of insurance" from the buyer's perspective is to pay a fixed premium instead of carrying risk. The value of the service, and thus the premium one is willing to pay, is determined by the risk being mitigated for each individual customer. The existence of "risk pools" is important for the insurance provider's cash flow, but a mere implementation detail to the customer. Insurance is not a subsidy or charity; if you and others like yourself are expected to cost more on average than some other group, you should expect to pay more. In a competitive insurance market those with higher risks will pay higher premiums; a company that attempted to charge everyone a flat rate would find itself either bankrupt or incapable of competing on premiums for any but the most expensive (highest-risk) customers.
Private auto and homeowners insurance are not price-insensitive, despite being mandatory (former case, by mandate, latter case, in practice for mortgaged property). If insurance companies paid out indiscriminately to the reckless; i.e. failed to manage their risk; then their premiums would get too high and they would lose their average customers.
There is no analogue for taxpayer-funded healthcare. There is no practical upper bound on the cost. The over-users will not run out of other people's money. Unless, of course, there are death panels; everyone loves death panels.
The same way responsible drivers should not pay for reckless ones? Or responsible homeowners shouldn't pay for reckless ones?
The whole idea of insurance is to distribute the costs so that no one has to bear long tail costs alone...