Well, like most organizations, they have an agenda to push and funds to raise, so any excitement they can engender is likely to benefit them and their PR should be read with that in mind.
Second, the actual figure is that 40% of all sites with "child abuse content" that they have chosen to "investigate" were in the U.S. That doesn't mean that the U.S. is a haven for 40% of all child porn.
We really don't know what standard they're using to judge abuse or why they choose to investigate one particular site over another.
Without them disclosing their methodology, the conclusions are suspect.
That being said, it wouldn't surprise me if a whole lot of child porn were hosted in the U.S. if only because there's a lot of broadband deployment here and a lot of zombie PC's.