Cuppla things- nobody has mentioned sea shanteys. Everyone sings them with an "English" accent. Most people don't know why they do this, they just do. Well, the shanty craze/ revival was fueled by two Brits- Stan Hugill and Louis Killen and everyone imitated them even though they were singing American songs (American crews historically used shantys far more than the Brits. In fact on British war ships shantys were forbidden- they used a fiddle or a fifer)

I remember Stuart Franke addressing the issue of accents on stage when he sang a Child Ballad in a Bronx accent.. Hilarious!!

Sometimes the "accent" is pronounciation of the dialect appropriate to the song.

And finally a quote from Gordon Bok- singing in a fake accent will give you lockjaw

Best- Julia