"Right now, American law bars the admission of aliens suspected of terrorist activity - but not of terrorist sympathies"

This quote by Richard Perle is describing the present laws in the United States concerning the admission of aliens. Perle is saying that while the United States law bars the admission of aliens presumed of terrorist activity, it does not bar the admission of aliens who are suspected of having terrorist compassions. This indicates that while the United States federal government is able to deny entry to those who are presumed of being involved in terrorist activity, they are not able to reject entry to those who are simply considerate to terrorist causes. This could be a potential security risk, as those with terrorist compassions may be most likely to engage in terrorist activity in the future. For that reason, it is necessary for the US federal government to be alert in keeping an eye on those who are confessed into the country, despite their presumed compassions.