When one language is used by two separate groups of speakers two varieties are bound to evolve. It is only natural and proper. No one variety is superior another (as there are no means to determine that in an objective way; personal taste, patriotism or snobbery don't count as objective). As for mispronunciations or any other derivations from what is considered standard: if enough people make them they become incorporated into the variety and regarded as correct.
Languages change all the time on all levels (phonetics, phonology, morphology, lexis, syntax, semantics etc) and a language that does not change is a dead language. British and American varieties evolve in two partly separate ways.
Also, linguistics is too beautiful a science to taint with politics.
Just my 2 cents. Or eurocents. Or pence. Whichever you prefer.