British Army officer arrested over military secrets leak

[Lyneham Lad]

but I don't understand why these figures should not be in the public domain in the first place.

a) The embarrassment factor, especially to our friends across the pond?

or

b) Giving aid and comfort to the enemy (to coin an old phrase). In this case, aiding the Taliban's propaganda campaign (or hearts & minds campaign, depending on your viewpoint).

or neither of the above!