This was first reported a few weeks ago - but it's worth telling the tale again in case anyone missed it. It's a great metaphor for the way basic freedoms and human rights are being eroded under this Government.
But hang on a minute, the argument goes, we haven't really got any choice. We are losing some of our freedoms, the freedom to carry an unsearched bag on to an aeroplane, say, in the fight against terrorism.
It's not a fight we started (so the tale goes - some would argue precisely the opposite, of course) - but if we're gonna win it we have to give a little.
Well, whatever you think of the premises here, this argument can only work if the human rights in question are only curtailed to the extent necessary to prevent some unspecified act of violent extremism.
Is this what's happening? Hardly: it seems the freedom to take holiday snaps in London is the latest quasi-terrorist activity to be banned.
Austrian tourist Klaus Matza and his son Loris were just acting like any other tourists when they took pictures of some of London sights during a recent visit. The trouble is, they aroused suspicion when taking snaps of buses and a bus station in Walthamstow and Vauxhall.
And now the tourists have said they had to return home to Vienna without their holiday pictures after two policemen forced them to delete the photographs from their cameras in the name of preventing terrorism.
Matkza, a 69-year-old retired television cameraman with a taste for modern architecture, was told that photographing anything to do with transport was "strictly forbidden". The policemen also recorded the pair's details, including passport numbers and hotel addresses.
As Matzka himself said soon after the incident: "I understand the need for some sensitivity in an era of terrorism, but isn't it naive to think terrorism can be prevented by terrorising tourists?"
"Google Street View is allowed to show any details of our cities on the world wide web," he said. "But a father and his son are not allowed to take pictures of famous London landmarks."
He said he would not return to London again after the incident.
"We typically crisscross cities from the end of railway terminals, we like to go to places not visited by other tourists. You get to know a city by going to places like this, not central squares. Buckingham Palace is also necessary, but you need to go elsewhere to get to know the city," he said.
He said the "nasty incident" had "killed interest in any further trips to the city".
Jenny Jones, a member of the Metropolitan Police Authority and a Green party member of the London assembly, said: "This is another example of the police completely overreaching the anti-terrorism powers.
"They are using it in a totally inappropriate way."