My memory and Wikipedia (http://en.wikipedia.org/wiki/Breast_milk#Composition) disagree. It about a magnitude lower than that. (In humans that is, of course. Not that breast milk is therefore bad.)
And that is so easy to check. Wonder what other tidbits of facts are wrong? Not that the gist of the article is therefore bad, but I think it better not to mention facts rather then mentioning wrong ones.
"Human milk contains 0.8% to 0.9% protein, 3% to 5% fat, 6.9% to 7.2% carbohydrates"
Excluding alcohol, essentially 100% of the calories of any food come from those three macronutrients-- 4Kcal per gram of protein or carbohydrate and 9Kcal per gram of fat.
9(3 to 5) / (9(3 to 5)+4((.8 to .9) + (6.9 to 7.2))) = 45% to 59%
Assuming the data in the wiki you linked to is correct, 45% to 59% of breast milk is fat, in nutritionist lingo.
Take 100 calories of lettuce (which is quite a lot).
Now take 100 calories of oil (which is not much at all).
Combine the two into a salad, which contains 200 calories and 50% of calories from fat.
Though I must say that "usually" doesn't really ring true to my ears. Perhaps it's the way nutrisionists talk, but is it really "usually" that way?
If someone presents a jug of a liquid and says "luckily for babies, liquid X is 50 percent fat", then I'd think that about half of the jug would be fat. Would most people where you come from think "half of the nutritional value of this liquid is made up from fat"? That's sounds impressively educated! Where do you come from?