Bit of over-enthusiastic hobby horse riding going on here, methinks.
The international dimension is certainly important, but ISAF is a NATO command, not a US one (although the US has huge influence, natch). There are plenty of non-UK/US NATO staff who could have kicked up a stink about this. But the Torygraph's editorial position makes it desirable to throw in the odd anti-American implication every now and then to try and widen its appeal. At the same time, it's not exactly been scaling new heights of journalistic authority recently.
So there are some particularly cross American officers. Who says? Well, the Torygraph said that someone did, so it must be true. And even if it is, what about the nationalities of the other senior officers in HQ ISAF, some of whom might also be more than a bit cross? "NATO generals in the Afghan capital Kabul are said to be furious about the alleged leak" doesn't quite have the same connotations, does it?
Plenty of heat but not much light generated by that single line.