The question posed to him at the time was “Why don’t you sing with an accent?” Reporters had noticed that, compared to the thick Liverpudlian accents the Fab Four demonstrated while speaking, they sounded positively American while singing. So do many other “foreign” groups, from Abba to Oasis to INXS.
Actually, linguists would tell you that the vocalists don’t really sound American as much as they sound “neutral.” Singing uses a different part of the brain than speaking; that’s why someone with a severe stutter (think Mel Tillis) is able to sing a song from beginning to end without so much as a stumble. That’s also why certain folks suffering from spasmodic dysphonia (such as “Dilbert” creator Scott Adams) can sing, but are unable to speak. In addition, singing can be likened to drawing; some folks have terrible handwriting, but are able to draw beautiful pictures because they take the time to pay attention to details. Likewise, a singer tends to concentrate on properly vocalizing the lyrics, “drawing” them out to fit with the melody.
Nevertheless, some habits are hard to break. In “Hot Legs,” Rod Stewart uses the British pronunciation of “vitamin,” and the Abba girls tended to pronounce the final “s” in English words with a hissing sss sound instead of the customary zzz. Country singers consciously maintain their twang because it sells songs about lonely truck drivers who’ve lost their girls to polecat outlaws. And don’t get us started on Elton John, who is regularly voted the champion lyric garbler in Mondegreen contests.