This morning, the U.K. Home Office defended its detention of Guardian journalist Glenn Greenwald's partner David Miranda under anti-terrorism laws. This is what officials said:

The government and the police have a duty to protect the public and our national security. If the police believe that an individual is in possession of highly sensitive stolen information that would help terrorism, then they should act and the law provides them with a framework to do that. Those who oppose this sort of action need to think about what they are condoning.

Let's turn this around: Governments that abuse the trust of their people by misusing the special powers granted to fight terrorism need to think about the backlash they risk provoking.

This is becoming a big story in the U.K. British officials detained Miranda on Aug. 18 at Heathrow Airport as he returned from a meeting in Berlin related to National Security Agency leaker Edward Snowden. Miranda was changing planes on his way back to Rio de Janeiro, where he and Greenwald live. He was questioned for nine hours, the maximum allowed under Schedule 7 of the U.K.'s Terrorism Act 2000, which applies only to such transit lounges. The law doesn't require police to have any reason to suspect the person they question, doesn't give the detained non-suspect the right to a lawyer, makes refusal to answer questions a criminal offense, and allows the police to temporarily confiscate and copy any data, phone records and recordings.

According to Miranda and Greenwald, the U.K. police asked about the Snowden case and about Miranda's life in general, but not about terrorism. I don't take it as a given that Snowden was well-intentioned, or that the U.S. and U.K. intelligence authorities are wrong to be concerned about the data that he stole (only part of it has made public, so who knows?). And it is quite possible that the U.K.'s security agencies were right to want to find out what information Miranda had, on grounds of national security or the integrity of intelligence-gathering operations. But there is zero evidence that any of this related to terrorism. At best, U.K. police were fishing for what might be there -- a bar so low for invoking special powers that it may as well not exist. The Guardian is suing the government on these grounds, among others.

It seems clear that police used the anti-terrorism law simply because it gave them the powers they wanted in order to question Miranda and download his data. That's what is disturbing here, and it doesn't just apply to the U.K. All around the world, governments -- especially since Sept. 11, 2001 -- have expanded the scope of their anti-terrorism laws to provide their security forces with more tools to foil terrorist plots. Some countries (let's just name Turkey and Egypt because their abuses have been in the news lately, but there are many others), use these laws in the crudest ways to suppress media freedoms, and to jail or intimidate opponents.

For the U.K., which arguably invented the rule of law in its modern sense, to do the same is disturbing on at least two fronts. First, it provides political cover to the repressive actions of authoritarian governments around the world. Second, it undermines the compact of trust that these anti-terror laws imply between nations and their governments. If voters cannot trust security agencies to limit the use of expansive anti-terror powers to their intended purposes, then voters can and should pressure their governments to withdraw them.

The U.K.'s Schedule 7 was already under review for amendment by Parliament. About half a million people are selected for inspection at U.K. airports and ports under the law each year, and 61,145 underwent substantive questioning of more than about 15 minutes from mid-2012 to mid-2013.

As David Anderson, the official independent reviewer of U.K. anti-terror laws says in his annual report on the subject published last month: "Nobody disputes that the ability to stop and examine passengers at ports is an essential tool in the fight against terrorism. Whether the power is proportionate in its current form is however a legitimate subject for both public debate and judicial scrutiny."

Anderson goes on to call for the government to review, in addition to changes already under way, the "need for a power to examine port and airport users without the need for reasonable suspicion," for it to "remain a criminal offence to refuse to answer questions asked during examination," and for police search powers to "extend to copying mobile phone records."

Given the Miranda case, Anderson's recommendations seem prophetic. Public pressure for change will probably increase. Anderson's detailed review of Schedule 7's use didn't find widespread abuse in practice, so this one instance may prove fateful. The opposition Labor party has lept on the story to attack the government, and newspaper editorials across the political spectrum have excoriated the Home Office over the case, with its self-evident implications for traveling reporters.

There is plenty of precedent for rolling back anti-terror measures that are found to be unnecessarily intrusive of civil liberties. Schedule 44 of the same U.K. law, which enabled the police to stop and search people in the streets without cause for suspicion, was used 250,000 times in the year 2008 to 2009 alone, but resulted in no convictions in its 10 years on the books. It was repealed in 2011, without any measurable effect on U.K. security.

That isn't proof that Schedule 7, a different anti-terrorist measure, also should be scaled back. It is, however, a strong argument that reflex government warnings aimed at stopping debate -- such as "people need to think about what they are condoning" -- should be tested to destruction.

(Marc Champion is a Bloomberg View editorial board member. Follow him on Twitter.)