As long as USD isn't shown on the order page, invoice, or receipt, I agree 100%.
It wouldn't matter if a page showed USD prices along with BTC. Paying in BTC implies being refunded the full/original BTC amount, no matter what USD prices are on the page. Anything else is a seller hedging value against the buyer.
by the same reasoning you can buy 1oz now, then come back 5 weeks later and say "BTC has doubled, send me another 1oz, ok"
not.