Its articles like these that lament people's basic misunderstanding of statistics. They use percentage of spam sent by countries in order to try to prove that spam is not being reduced in the United States. The problem is that simply relaying a percentage of total spam does not prove or disprove this point. It simply shows whether the US is changing more or less in proportion to other countries.
Did the total number of spam messages go up or down? What about the total number of bot nets? The reality is that the total numbers could have gone down, and the US percentage still could have gone up depending on whether other countries went down further than the US.
Percentages always add up to 100!