(On the other hand, if large companies will accept it in payment, they've probably got an idea that a given currency will be around next month or next year.)
I don't think this is the most likely answer. Most likely is that the big companies have an instant exchange set up where a purchase made in bitcoins is immediately converted to dollars, and they charge their customers a small transaction fee in the form of an exchange rate difference.
...Someone from the back row shouts out "Because our AdSense profile has determined you were visiting websites about cigarettes recently, your health insurance premium has gone up by 5% and you will probably die slightly sooner. Remember, [i]f you have something that you don't want anyone to know, maybe you shouldn't be doing it in the first place!"
Is it cynicism if you're just using a Markov chain to predict what other Slashdotters will say?
(Although obviously this is auto insurance, so I'm sure someone can translate the threat appropriately.)
"No matter where you go, there you are..." -- Buckaroo Banzai