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.
Unlike some other ASIC vendors, BFL products are priced in $US not BTC, how you come up with those $US is up to you. If you choose to use Bitpay instead of Paypal because you want to convert some BTC to $US to pay for the item, then what happens to the BTC after Bitpay receives them is no longer your concern. Bitpay might convert them to any number of different currencies at the request of BFL, there are many currency options Bitpay offer merchants. If I was BFL I wouldn't leave the money in BTC, far too risky, I would use fiat, since BFL need to buy wafers, other components. pay rent and wages with it, can't suddenly have the value dropping by 50% as would have happened to any payments in the week after April 10th. But once again what happens to the BTC after Bitpay accept it is none of your business. All you need worry about is that you get the item you have purchased in what you feel is an acceptable time frame, or seek a refund of the $US purchase price.