Because if non-U.S. cloud providers are also active in the U.S. itself, the U.S. government might still be able to gain access to the data stored at these foreign providers.
Dutch language report:
http://www.surfsites.nl/cloud/download/Clouddiensten_in_HO_en_USA_Patriot_Act.pdf
Dutch language news articles:
http://www.security.nl/artikel/43466/1/%27Gegevens_niet_veilig_in_Amerikaanse_cloud%27.html
http://www.nu.nl/internet/2933102/sp-wil-opheldering-amerikaanse-toegang-nederlandse-cloud.html