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One of the questions on a visa application is something like: "Do you plan to attempt to overthrow the Government of the United States by force or subversion?".

Political dissidents and crackers may have an issue there, if that's their stated purpose.

I always enjoyed Peter Medawar's response: "No, and if I do it by accident, I'll be very sorry"

Sure, but that's a different issue entirely.

The cases that spring immediately to mind in security tend to have more to do with the fact that conference visitors are conducting business in the US than out of any particular concern with the subject matter. People come to, say, Black Hat, and in addition to speaking also have their names attached to training courses.