I remember a judge on a British TV talent show criticising a contestant - white British - for singing in a Jamaican accent; however, none of the other contestants were criticised for singing with American accents.

A matter of perspective. This sort of "crossing", in that it appears to involve a racial leap, may be too much for them. Or it may have been done badly (as compared to the American accents, which were done better).

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My example again is that there are Japanese, Germans, Italians etc. that are performing in full on Jamaican accents and dialect and hardcore Jamaican music audiences have respect for them. The point of my example is that it is not the default position that one must perceive "singing accents" as phoniness or betrayal of one's prescribed identity. Taking an ethnocentric position of "We are English, so we shouldn't sing these horrid American accents" doesn't help much understand the "Why?" These artists show their competency through using appropriate accents. It is not necessarily a sign of their being passive, weak-minded slaves to media, cultural hegemony, low esteem etc etc, but rather can be active expression of identity and taste.

I am fully secure in my local identity. In fact, if anyone gets me started I am happy to go on for hours about the nuances of my regional accent -- I am consciously aware of them. I think that where I am from is the greatest place in the world! But when I sing I will select whatever accent I feel is appropriate to 1. the aesthetics of the music 2. the identity I wish to convey. When I go into a coffee house in California I will ask for a 'smawl khawfi' in my usual way -- Damn them if they have to say 'What?' You've got a Southern New Englander in the house; step out of your damn state for once in your life and get some exposure to others. :) When I sing a song tho, I may want most people to understand the words the very first time, and allow my region identity to fade into the background.

It can also be really fun to sing in different accents; speaking in them may be considered inappropriate, but since singing in them may not be, it offers a chance to shift into that character in a 'framed' context. I am offering this in answer to the question of "Why?"