This is another one of the British/American English grammar questions to which the only answer is “They do it because they do it.” There’s no deep reason why the British do it one way and the Americans do it another way. Most choices in grammar are arbitrary, so it’s not worth the time to debate how such choices were made (and it’s an immense waste of time to debate which choice is better).
It’s my opinion that the British way of doing this is deeply and offensively wrong. Like putting an “R” in the middle of the word “drawing”, it grates on my ears.
The “official” position is apparently when the group is referred to by its members, you use the plural, and when it’s just the organisation, you use the singular. However, in practice nobody does this: they use the plural almost exclusively. I spend a lot of time correcting copy in which this usage stands out to me like a paedophile on a bouncy castle.
Get deeply offended by the mundane facts of the world if you want, but I hardly see the point in it. As Wendell Wagner said, there’s really no point debating which way is better; they’re just arbitrary differences, with some speech communities having gone one way and others having gone another way. You can try to argue that one is more “logical” than the other till you’re blue in the face, but up against the mass of other “illogical”, but uncontroversial, aspects of language, it’s just a drop in the bucket.