Confirming and expanding on points already made: Treasury imposes these civil penalties on Americans who travel to Cuba without permission. The standard fine is $7500. The standard of evidence is not the same as in a criminal case. Civil fines can be imposed much more easily. The TSA does the same thing, by the way, to people who attempt to smuggle prohibited items or "interfere with screening."

Believe it or not, the U.S. government stations people at Canadian airports to spot Americans disembarking from airplanes coming from Cuba. They get those people at U.S. immigration, which is physically in Canada. They just follow them within the airport. It's like shooting fish in a barrel!

Bruce