>I wonder if this is unique to the culture of France, or at least Paris, that they do not like to hear their language from a foreigner? Maybe the language is part of a group identity, used to identify locals who can be trusted to follow local customs? It was a little saddening.

I'm sorry to hear that. As a Parisian, I don't think this had anything to do with protecting an identity, rather we're just as happy to practice our (often terrible!) English as you are to practice your French!