First, the vast majority of American colonists were from England, so how exactly is English not their language.
Second, do you take the same offense to Commonwealth countries that have totally ruined English as well?
Third, it is commonly accepted among linguistic experts that neither modern American English or modern British English are anywhere close to original English.
In other words, languages evolve. It's not a hard concept to grasp that two countries with the same common language at some point 300 years ago but separated by an ocean would linguistically (and culturally) evolve independently of each other.

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