Following your logic, s/bitcoin/Euro/ would mean that the Euro is not a currency. That's just plain goofiness -- the USD is just the default currency at this establishment.

The logic seems to say more that the owner is not treating it as a currency. You can't really define something as a currency or not based on one person's actions.

No, only because Bitcoin isn't stable enough right now to know what the price should've been in BTC.

You mean yes, not no. It's precisely because bitcoins are not stable that bitcoins are not a currency.

You should try living a country with high inflation. People lookup exchange rates every single day.

And those people do not willingly use that currency - they are forced to by their government.

Whatever currency it is that they are comparing to is the currency they really want to use, and probably do on the black market. I'm also confident that that is the currency they keep (or try to keep) their savings in.

And you see real currencies being used in many of them as black market currencies instead of the national currency. You can also see countries trying to stop inflation by indexing to other, stable, currencies.

That is not the problem of stability, that is because the restaurant is paying in USD to their contractors.

This should change in the future.

If you're trading Bitcoins for a meal, I don't think it really matters if it fits the dictionary definition of a "currency."