Gobsmackingly

Working on a book that's been translated to American English from Australian.

Some ridiculous phrases:

reefed it
gobsmackingly
to be suss on
fringe
retired holidaymakers
in his swag

What I've never really understood about this, especially when the narrator is not American, is why do we Americanize the British/Australian phrases/slang? Besides changing their obviously wrong spellings and grammar, why do bother? Any thoughts?