My guess is that they're people who simply have no ear for language.

I'm not convinced. I have the pronunciation problem that Fox is talking about, but my pronunciation of French and German is pretty good, considering I'm nowhere near fluent in either at this point. I can, for instance, hear the difference between "dessous" and "dessus," and "attendre" and "entendre." But in English I know most of my vocabulary comes from reading, and if I haven't heard a word spoken, I'm really bad at guessing how it's pronounced.

And just for completeness, I'm very, very left-brained, and I have a good understanding of grammar rules; I don't go by how things sound. Make of all that what you will.