Treason doth never prosper: what's the reason?
Why if it prosper, none dare call it treason.
Sir John Harrington, Of Treason--Epigrams (bk. IV, ep. V)
In a sense, one can see where British authorities are coming from. The linchpin of treason is the allegiance a person owes to the realm. Treason is a crime, fundamentally, of traitorous intent. By making treason part of the law, a society announces that loyalty to country is a norm the departure from which — if it involves aid and comfort to an enemy — is an offense of the highest order. By enforcing that law, government renews that announcement, and society reaffirms the bond between the individual and the body politic.
Note McCarthy's phrasing here - Loyalty to country, not government. So all of the "Criticism is the highest form of patriotism", et nauseum, are kindly asked to shut up and keep reading.
Here, however, the norm has atrophied. Britain has failed to cultivate national loyalty as a fundamental aspect of Britishness. It has, instead, permitted a virulent strain of anti-British hostility to thrive in its midst. With such impunity, this enemy has unsurprisingly progressed (as its numbers have increased) from merely talking about jihad, to exhorting its execution, to, now, carrying out missions under its banner.And that, almost four years after 9/11, is where we stand in the War on Terror, or whatever we call it these days. Despite the fact thousands of individuals have sworn to destroy us, to kill as many of us as possible, we can't even agree we're at war.
But most militant Muslims in the U.K. would no doubt say, quite convincingly, that they can’t be deemed traitorous because they were never loyal in the first place. The nation has failed to instill a sense of allegiance as a reciprocal obligation of enjoying the amenities of British life. For their part, the Islamists never pledged fealty ...jihadist loyalty extends only to the cause of establishing a worldwide Caliphate that hews to jihadist principles. For the militants, Britain has always been the enemy...
...In discouraging treason prosecutions, Lord Carlile, who reviews antiterror legislation, summed up the point, urging caution because “[t]reason tends to apply to war between nations.” The subtext is clear: An international terror network is not a nation, so adhering to one may not be treasonous. This, of course, is but a short distance from the argument — oft-repeated by opponents of post-9/11 military operations — that the current hostilities, which have resulted in thousands of military and civilian casualties, do not constitute a war because war, too, implies a conflict between nations...
...Moreover, if it’s not a war, then the laws and customs of war — such as those that permit detention of unlawful combatants because their uncivilized warfare targets civilians — do not apply. In that case, so the argument goes, the terrorists are mere criminal defendants, entitled to all the privileges that implies.