Via The Kept-Up Academic Librarian:
"Since passage of the USA Patriot Act after Sept. 11, 2001, a number of academics have been denied visas or had them revoked. Universities and scholarly associations have written letters to the government to no avail."
So foreign scholars are being kept out of the U.S. seemingly because some in the government are worried about what they might say (link to Christian Science Monitor story) to us Americans. The DHS claims that these academics are kept out because the Patriot Act's section 411 gives the government the power to exclude anyone who espouses or endorses terrorism.
How long before we librarians are given a list of illegal books -- books that are said to espouse or endorse the viewpoints of terrorists? And since the t-word has such a broad definition, I reckon we'll all have lots and lots of weeding to do.
See also: Patriot Act in the U.K.