I suspect they have also run numbers and concluded that this is probably within parameters for a normal bad year. We know 80% of those who get it will have mild symptoms, and we also know not everybody will get it (they probably have good numbers). As such the goodwill from accepting a bad year is worth more long term than the savings from not covering it. They may have also factored in future bailouts which they expect to get if things get bad enough.

Insurance companies are all about numbers and are very good at running them. they are not always right, but they are right often enough for their purposes.

That may be the case, but I would guess other insurance companies will buckle under similar pressure. Perhaps I'm being naive, but it seems like the worst kind of PR nightmare to financially cripple a group as big as those who will eventually be infected.