I think in part it's due to very few native English speakers being multi-lingual. Rudely correcting someone on their second-language is a faux-pas for most people in the UK. From my experience, people will try to be helpful and constructive with their criticism - because lets be honest, how many native English speakers could say much more than Hello, Please and Thank You in anything but English?

Conversely, I can see why some Europeans might be more passionate about correct grammar and pronunciation. As youth begin to speak in a hybrid of English and their native language, it begins to erodes their cultural identity. It makes sense if they're passionate about their countries heritage, which many French people are, to try and resist the Anglicising of a part of that.

Americans butchered the English language anyway so you couldn't do much worse ;)