I've experienced the same thing with older folks. Another thing I always thought strange, is seeing to Cajun French speakers switch to English after a few sentences in French. It's possibly for our benefit(non-fluent) or maybe just habit. Like acknowledging the background of the person you are in conversation with and then switching to what is most familiar (english).
So... How's a guy like me to learn French. I do have friends,(native Cajun French speakers) who have been a big help, but I've had to give them a little prodding. Oh, well. I put all my hope for the continuation of my French heritage in my 10 month old son. Thats what emersion programs are for.
Layne

I've found that the people who know what I am doing make an effort to speak in French. But most get frustrated pretty quickly if you miss something. And like you said, they usually instinctively revert back to English.

Lots of places in Canada bilingual french native speakers who through education or travel have gotten used to hearing & speaking to people with different french accents are at ease in speaking french with non native speakers. But there are others who having remained in their local community, switch to English at the least hint of a different accent, especially in Montreal where their is a long ingrained behaviour to accommodate the english speaker who in the past was usually not bilingual. Now the young people of both linguistic backgrounds have indetectable accents in either language, having studied from day one in french. The older english speakers like myself who learned french later and still carry an english accent , even a trace will often encounter the behaviour you describe.
Rhis is frustrating for french speakers from other parts of Canada , like Acadia who although just as fluent as Quebecois but have a different accent will also get answered back in English. Sometimes they take is kind of like an insult, but the person doing it was unconscious of what they just did because it's a pattern.
Some of my Quebecois educated friends call it "une esprit d'assimmilé"
It deffinitely presents an challenge for those interested in exercising their love of the language.
Last week week I called a plummer on the phone and as he was out, spoke to the secretary for about 5-10 minutes, describing the problem (extending a heated water line into the lake for winter water access for a cottage) . She understood everything I had asked and had answered, then suddenmly must have noticed my accent and asked me if I spoke french. and wanted to continue in english . I answered "voyons donc, ca fait juste 10 minutes qu'on jase puis tu sais pas que c'était en francais qu'on jasais?