David,
I have every sympathy with your situation, and have sometimes witnessed the reverse snobbery you refer to. Indeed years ago I might have been guilty of it myself. However, many of what we nowadays call folk songs originated in the theatre and the refined pleasure gardens like Vauxhall and Ranelagh in London. Another valid point is that the folk scene in Britain is definitely a very different beast to what went on prior to say 1900. We are all reviving something and most of us are arranging the material in a very different format to what it existed as prior to 1900. Personally I have little quibble with trained voices singing folk music. However certain songs do sound somewhat incongruous sung in this way, sea shanties as just one example. I entirely agree with your final comment. Rudeness in any form is unnecessary.