That means they have a net exposure to the bitcoin price (possibly quite a large one). From their point of view that's not all that desirable.

Except that, with the price rising, they're making money from that "exposure" on average.

And they obviously believe the price will keep going up, because they're a bitcoin business, and believe in bitcoin. If the price stops going up, their company will fail anyway.

And finally, perhaps you noticed that coinbase is processing thousands of bitcoin purchases now. So I'd say, with all that data, it's not actually that risky for them. If the price starts going down, they'll know it first, because they'll be the ones making the purchases. :P

The exchange rate has no effect on Silk Road. People will still want to buy BTC.

That actually sounds more risky? Isn't there a risk the money won't clear?

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.