Perhaps it's analogous to "register" in speech, where vocabulary and accent change according to the context. Many singers seem to assume, consciously or unconsciously, that an American (or more often pseudo-American) accent is what one uses for singing, and it's different from speaking. I am not too comfortable with that even when it's a Brit singing an American song, and I hate it when it's Brits singing songs they've written themselves.

There's often something similar with "trained" singers, who enunciate their words in a completely different way from how they would speak, not just for clarity (which is fair enough) but with quite different pronunciations.