"legal tender" is a strange concept. For example, £5, £10, £20 and £50 notes are not legal tender in Scotland. But I don't think most Scottish people consider those notes to be "more like a commodity than money."
I'm not sure what happens if stolen Euros or gold are accepted as payment. These aren't legal tender in the US, so if they are exempt from the aforementioned rule, then bitcoins might be as well.