Are you sure your source said “England, Ireland, and Scotland”? Because it would be more accurate to say “(parts of) England, but not Ireland or Scotland”. We still pronounce our final “r”.
Mea culpa. You’re right, I just went back and re-read it:
A very large change took place in some accents of England that seems to have started in the seventeenth century. Speakers in parts the south and east of England started to pronounce /r/ only when it was followed by a vowel. This ed to changes in the way the vowels were pronounced. This change has spread over most of England, and is also found in accents (like Australian, Singapore, and New Zealand English) which developed from English accents of the last 300years (in these accents ‘sauce’ might be pronounced the same as ‘source’ and ‘spa’ pronounced the same as ‘spar’). But accents which developed from English accents older than that (such as most US accents of English) still pronounce /r/ at the ends of words and before consonants. Because this is such a large change, the accents that have kept this ‘post-vocalic r’, like most kinds of US English, Scottish English, and Irish English, seem more like accents of the seventeenth century than do those of accents which have lost the /r/.
Anyone ever listen to Herman’s Hermits? Peter Noone and his cohorts never tried to hide their British accents!
Every modern British rock band (i.e. from the 90’s until today, and many from the 80’s, e.g. New Order, the Smiths, Joy Division) I can think of sing in their native accents. In fact, I cannot think of a “new” British band I’m familiar with that sings in an American accent, and there seems to be a definite shift away from trying to sound American, or even stereotypically English, to using heavy local accents when singing (see, for example, Arctic Monkeys and Glasvegas).
I think the answer to this question is the same as the answer to the question why e.g. The Killers sound so British: because they think it sounds cool, and because they think it will sell more records.
It’s funny. I always assumed that the act of singing, because it’s such an artificial way of enunciating, stripped away accent variations, and that’s why British acts sounded unaccented to my American ears. I assumed that James Taylor, say, sounded just as unaccented to British ears as Van Morrison did to mine.
Now I find out they were deliberately singing in fake American accents. Weird. But then, being American is weird in general-- it’s bizarre when you find out how much attention people in other countries pay to your culture, when you know nothing about theirs.
I once deeply offended a Japanese girl by starting to explain to her what the Academy Awards were. She acted as though I was explaining that the world was, in fact, round. But, hell–* I* certainly don’t know what the big Japanese movie awards are called; why would I assume she knew about ours?
thetruewheel, I hope by reading this thread you realise that “unaccented” in that context actually means “sounding similar to my own accent”.
As a country you’re (usually) bigger that other countries, your global media are definitely louder than other countries’, and (in my experience) your citizens care less about other countries - possibly due to the first two…
And in response I might say “Why do American singers lose their accents when singing?”
The act of singing distorts regional dialects; singers generally sound a lot more similar to each other (accent-wise) than speakers do. An American has a much stronger accent in speech than when singing.
I know, we’re total dicks! I don’t know why you keep encouraging us; it only makes us worse. 
I know that there is an Operatic semi-informal standard. Is it possible that some of these individuals have had at least some voice training, which teaches how certain vowels and consonants should be intoned? This could account for the loss of accent. The difference in accent from a Russian, Japanese, and Italian soprano is not very noticable. How this would apply to popular music, including rock and pop, I don’t know for sure. However, if they had traditional voice training, it would affect their singing accent.
Also, emulating the originating style can cause this. Foreign singers who are singing American Bluegrass generally have the Bluegrass “twang”. It’s integral to the sound of the piece.
No, no, my accent, which is utterly non-existant, is the most neutral accent.
There are early advertisements and posters for upcoming performances that used the “Beetles” spelling. Another supposed inspiration was the name of the rival biker gang, whose leader was played by Lee Marvin, in The Wild One. As with anything of this nature, any or all of the theories could be true, and it probably doesn’t matter much. It was just something that four or five guys in an unknown struggling band thought was cool.