A Day In The Life Of A Spammer
Because spammers go where the bandwith is.
From an interesting article with some insights about the reason why most spam is US based:
"The United States is the origin of choice for spammers, said Alperovitch, because of the plentiful supply of cheap high-speed bandwidth. "Spammers need big pipes, and they don't want to pay much for it," he said.
That explains the low percentage of spam messages originating from overseas' IP addresses. The lack of cheap bandwidth outside the United States is stymieing spammers' attempts to scale up the volume of their mailings to U.S. sizes."