Have you every tried to place an order with BFL in BTC? It takes you to Bitpay which immediately converts your BTC into USD and pays BFL in USD at the current exchange rate. So these claims are bogus.
You don't know that. Even if Bitpay pays out their merchant in EUR or USD, the merchant can still opt to convert some of it into BTC for the long term.
So Bitpay has no option to pay out the merchant in BTC? Seems like a lack of service...