Other people do pay for the cancer treatment. Make up a pool of, say, 1000 people. Make up a cancer rate of say, 5/1000. Make up an asthma rate of say, 50/1000.
The people that are going to get cancer are unknown, but the whole group is willing to pay 0.5% of the cost of cancer treatment for a contract that covers 100% of the cost.
The 950 people that know they don't need expensive asthma treatment don't really want to pay for contracts that will cover expensive asthma treatment, so (in a pure insurance market) either the cost has to be included in the contracts for the 50 that do have it or the treatment can be excluded from the contracts.
