I agree with you that there was an overreaction. But that's a risk each of us runs when we publish something via a public medium. Any time I want to publish something, whether it's a private SMS, a tweet, a comment on HN, or something else, I'm obliged to consider its content, and make a judgment call on whether it's appropriate. If I'm wrong, I have to deal with it.
Here's what "should" have happened:
<sacco>: <original tweet>
<public>: Hey, that's really racist.
<sacco>: Oh, I didn't mean it that way - I'm making fun of American insularity.
<public>: OK, I still don't like it, but I get what you meant now.