In some countries, if you post something that is critical of the Maximum Leader, the police come around and suggest that maybe you are psychologically disturbed. Maybe they mention this idea to your family and your boss. Maybe they 'suggest' you'd like to retract your critique.
Amazingly, that's the United States today: When Bears Growl (Or how I become the subject of a Secret Service Investigation).
I don't object to the Secret Service following up on complaints. And if the subject of the complaint is web art on “Bush and guns” I don't think it is unreasonable to go and visit the author to see if he seems like someone likely to engage in violence.
But if the account linked above is accurate, then I think its fairly clear that somewhere in the investigative process the Secret Service crossed a line.
Bad apples or agency policy?