Do you know what makes me chuckle?
You ask a question, in which you specify should posts be removed if they're negative about Americans, and in response - the Americans all come out speaking about the "first amendment", as if the whole world follows their laws...
In answer to the question - there should not be a blanket ban, no. Sometimes criticism is valid. So when people criticise American isolationism and cultural ignorance - they can often have a point, and can often back it up.
Note to all people - if you want to make a universal statement about freedom of speech, why not use the Universal Declaration of Human Rights (article 19, I think), which at least has some scope outside of your own country?
There's a big difference between fair criticism based on a common observation and just spouting nonsensical abuse, like the loathsome "hurr, Americans are dumb, hurr" which is so fashionable nowadays...
Often the best response to an opinion we disagree with is not to silence it, but to respond and explain why it is incorrect. obviously, this only works with a point that's rational to begin with, and tends not to work so well with prejudice.
We can't ask all free speech to be intelligent (some people just aren't) and can't ask that free speech be inoffensive (you'll always find someone who's offended by something!).
What we can as is that people use free speech responsibly, and don't abuse or degrade others. If someone isn't capable of censoring themselves according to this respectful principle, then sadly someone will have to do it for them, to protect those who may be victimised by such hate-speech.