The Beatles knew which market was important! Well, Paul McCartney did at least
The accusation of them singing in American accents is something I've heard before.
But if you consider which side of the Atlantic many of their influences are from, then we're back to the DMZ situation. Where did you hear something and in what context? Rock & roll, blues etc was 'yeah yeah yeah' not 'yes yes yes'. If the Brits had gotten (or got, whichever you prefer) there first then maybe it would have been a different story