All other countries fell under 10% individually, which makes the numbers even more surprising. With the amount of corporate regulation that the U.S. has on the books, you would think that companies would be less inclined to setup shop on a U.S. server, and that hosting companies would be more aware that they are home to such content. Clearly that isn't the case.
This isn't the first time in recent history that the U.S. has come in number one for malicious web content. A little over a year ago, Sophos released a report indicating that the U.S. was also number one for spam, responsible for more than a fourth of spam relayed around the world.