Buying something with BTC doesn't mean it's priced in BTC.
With the possible exception of dark web drugs/gun/murder-for-hire/WHY every case I've seen of BTC being used for goods and services involves dynamic currency conversion. i.e. the price is in, say, USD and the amount it costs in BTC is derived from the USD spot price of BTC.
It's like I swipe my credit card in Japan; the price is in JPY but I pay in USD.
I would be interested to see any counterexamples.