So then they are stuck with bitcoins and you get a chargeback fee. No big deal if the value keeps going up. They sell bitcoins anyway, at a market rate plus a percent.

There is a risk that the price goes down, but the bitcoins don't drop out of their pocket because your bank refuses to clear a transaction to your account.

To be a responsible / sane business, they need to keep a hedge of Bitcoins anyway, in case the value spikes and they can't immediately buy the bitcoins for the price that they quoted you, that you locked in when you ordered; that is another factor contributing to their risk of loss as well.

The part that makes the business stable is that 5 day lag between their purchase and sending of your bitcoins. Their losses are limited to whatever can be stolen (now mostly some other way besides bank chargebacks) and whatever swing as described randomly falls above the level of their margin.