As I noted in my last comment on this incident, in another HN thread,
http://news.ycombinator.com/item?id=3529500
"a foreign national kidding around about a trip in which he or she will 'destroy America' shouldn't be surprised to be questioned about that by law enforcement officers." It is routine national policy in every country of the world to give border officers full discretion to deny entry to any foreign person, even if the foreign person has a visa issued by the host country's overseas diplomatic officials, and even if the foreign person comes from a country with a visa-free entry treaty with the host country (as the United Kingdom has with the United States). A general policy of visa-free entry or routine issuance of visas to visitors who meet defined criteria does not remove an immigration officer's general discretion to deny entry to any foreign visitor, without further legal recourse.
I encourage Hacker News participants to check the law of the country where they live, whatever country that is. Very likely you will find that immigration officials have exactly this kind of discretion, unreviewable discretion, in your country. A foreign visitor who kids around about engaging in any kind of illegal or bothersome activity, even the kind of activity that many loyal citizens of the host country engage in, is at risk of being denied entry in pretty much every country in the world. The way to test how often this kind of rare edge case occurs in other countries is to try the experiment--tweet about the fun you desire to have in the next country you plan to visit, in the same terms, and see what happens. Probably most British visitors to the United States who previously have tweeted similar messages have been allowed entry to the United States, without anyone taking notice of the tweets. And probably on some future occasion someone else may tweet the same kinds of statements, and not be detected. But try it yourself if you wonder what other countries might do. In general, each country of the world gives its immigration officers unreviewable discretion to deny entry to foreign persons for any reason or no reason at all.
On the broader issue of whether or not this lone incident is a sign that the United States no longer cherishes freedom of speech, no of course not. Here we all are talking about this incident, with many criticisms of the United States being openly expressed. I daily exercise my right as an American to criticize government officials at all levels in the United States. I will keep right on doing that no matter what I hear about practices by immigration officers at United States borders.
Moreover, the United States continues to enjoy substantial net immigration and a large number of asylum claims by people from other countries who expect to enjoy more freedom once they start living in the United States than they did in the countries of their birth. Here in the United States, I can disagree with you, and you can disagree with me, and we can be civil about that, and not be afraid of secret police or private militia hit squads coming after us if we express a controversial opinion. Other countries also provide the benefit of free speech, and people who are concerned about recent trends in United States law are correct to be wary about granting the government intrusive authority to monitor the private conversations of people in the United States. But the reported incident, while perhaps an excessive response based on the facts reported so far, does not suggest that the United States has lost free speech, nor indeed does it even suggest that the border response to such a tweet is a response that would not be found at other national borders.
The continued interest here on HN in founder's visas and in other efforts to loosen immigration requirements is evidence that there are still plenty of people around the world of high levels of education who would be happy to settle in the United States and pursue their careers there. There are still people who advocate that all of the tens of thousands of university students from dozens of countries who attend undergraduate or graduate classes in the United States should gain residence visas when they gain their degrees. There is plenty of demand to reside in the United States, to visit the United States for tourism or for business, and to invest in the United States. That is not going away any time soon.
Believing in free speech, I welcome your kind comments to this comment, and will read them with care to see if I can improve my thinking on this issue.