One of the more acute contributions to the soul-searching about the disappearance of three London schoolgirls came from the former foreign secretary, William Hague. Asked on a weekend television talk show why the girls had not been monitored and prevented from leaving, he remarked that questions about state surveillance usually came with the implication that there was far too much; in this case, the implication was that there had been too little.

Nor is this the only contradiction to be highlighted by the departure of Shamima Begum, Amira Abase and Kadiza Sultana to Turkey – with the aim, it is presumed, of joining Isis fighters in Syria. There are calls today for Twitter and other social media services to do more to shut down websites used to disseminate Isis propaganda and aid recruitment. One of the three girls was reportedly signed up to 70 such sites.

But to those who argue, in the wake of the girls’ departure, that such measures are necessary – even obvious – I would say: not so fast. Barely a month ago, François Hollande and dozens of national leaders walked through Paris at the head of an enormous crowd in defence of free speech. It has not taken long for that united front and all its certainty to start unravelling. How much free speech can we really bear?

It has not taken long for the united front of ‘Je suis Charlie' to start unravelling. How much free speech can we bear?

The desirability (or otherwise) of state surveillance and the limits (or not) on free speech are extraordinarily difficult questions. But there are easier questions relating to the three girls from Bethnal Green Academy, questions to which it is possible to hazard answers.

Some have asked, with incredulity, about the appeal to young British girls of Isis. Is it really so hard to discern the attractions of a journey, especially one with an altruistic and religious purpose, to girls who may have led very sheltered lives? It will sound insensitive, but the close embrace of their devoted families may be part of what these girls, and others who have made the same journey, sought to escape. A teenager who regularly accesses dozens of Islamist websites unbeknown to her family is living, to an extent, a secret life.

Yet how far should the state restrict individuals’ travel because the authorities disapprove of the traveller’s purpose? One definition of a free country is the right to leave it. Is possessing an air ticket to Turkey to become a suspect activity in itself? If so, you can be sure that other, more convoluted, routes will rapidly be found. To those Britons wanting to join Isis, I would say: let them go. This is more a family matter than a cause for national breast-beating, and I see no reason why police officers have to be sent, at great expense, in an effort to rescue these girls from themselves.

There was a way, though, these journeys could have been stopped, without the presumed purpose being a consideration at all. Three girls, well short of adulthood, were able to pass through a major British airport and board a plane for foreign parts by themselves. One reason, perhaps the chief reason, why this was possible, is that the UK is almost alone in having no exit controls at its ports and airports. There are people, or now often machines, whose job is to match passports to tickets; there are people who scan your hand baggage; there are stewards on planes who check your boarding pass. But there is no one whose job is to check your ID for your age and nationality and ask, say, whether your parents know about your trip.

The reinstatement of exit controls has been on the agenda of this government, but it has been subject to seemingly indefinite delay. Just as it can be argued that something as banal as stricter airport security could have prevented 9/11, so proper exit controls might have stopped Shamima, Amira and Kadiza. This is something that can be addressed far more easily than a teenager’s wayward dreams.