I do agree that it is better to sing in your own natural accent but I do think Tunesmith is being just a little intolerant.

Tunesmith has pointed out one reason himself. You get so used to hearing songs sung in an American accent that you subconsciously pick it up and adopt it as part of your singing voice. This will be especially true if you have been listening to such songs from an early age when you very quickly pick up accents.

My normal speaking accent is Northern English and most of the time that is the accent I sing in but when I sing Scots songs people here tell me I sound Scots. That is probably because I spent four of my first seven years in Scotland. One person even identified it as Aberdeen which is where my mother came from and where I was born. It is not something conscious, though I do try to pronounce the words correctly.