Those convert the coins to USD I'd imagine, and do it on the spot. The price of bitcoin skyrockets and you expect to get that gain even though you willingly parted with them for a product at the value of the time? I'd ask, why? Its no different to making a trade on Gox and then seeing the price go up.
Your logic has major discrepancies.
1. BitPay is coverting BTC to USD on behalf of BFL and to the extend BFL wish. They may covert 100% of the bitcoins received, 50%, or none at all. Depends how BFL setup their merchant account with BitPay. Don't forget that contractual obligation to covert all of the BTC or part of it is between BFL and BitPay, not between BitPay and BFL customers!
2. If you ask why, I'll ask as well. If bitcoin price skyrockets, I don't understand how is purchasing bitcoins from MtGox and sending them to BFL is more profitable than purchasing bitcoins from MtGox and keeping them in my bitcoin wallet? On the contrary, if bitcoin price skyrockets and BFL refunds BTC pre-orders in USD they simply profit from the free option call gambling customer money.
3. If bitcoin price drops to zero BFL customers are losers again despite that they've some chance to get USD priced product in future. It is very close to mind that if bitcoin price drops to zero the price of BFL ASICs will be zero either for they can not be used for anything else but mine bitcoins. In fact you'll have to pay extra money to dump them in electronic garbage depo.