Slippery slope?
Presumably the MoD would like to maintain the option of not disclosing how many aircraft/ships/personnel are in a given theatre. Just because they currently do disclose how many manned fighters are operating in the area doesn't mean they might later decide it's a bad idea, or the possibility of lying about it to confuse an enemy's intelligence estimates. If they hadn't fought the FOI request they may have set a precedent that would be hard to back down from.
Incidentally although ISIS don't have a counter-air component, that I'm aware of, that doesn't mean they don't have the means to destroy the drones. Vans seem to be a popular attack medium at the moment, so the less information they have on what's operating where the better. I suspect the manned aircraft numbers are only widely available because there are people who stand at the end of runways counting these things anyway.
Biting the hand that feeds IT
