My next question is, with some exceptions, how and why is it that British singers sing with American accents?
Why do some American singers sing with a British Accent?

Like Billie Joe Armstrong...I don't know WHAT kind of accent he sings with but he's from California

2. It's a strange phenomenon for some British singers to sing with American accents, but it's something that's cultivated over time.
Ozzy Osbourne has always sang with a vaguely American accent though...
Truthfully I think it has less to do with development over time and more to do with how easier it is to sing with a certain accent for certain music. Most british punk bands sing with incredibly strong british accents.
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