That's because health and disease don't work according to commercial rules. To take a current hot issue: as researchers discover genetic predispositions to disease, insurance companies want to screen for them and charge higher premiums. From a business point of view, this is only common sense. From a human point of view, it's cruel discrimination against the sick.
and if they can't charge higher premiums for the sick, they must charge higher premiums for all, which means me, which means they're discriminating against the healthy because i almost never use medical services