United States Boundary
This is an Unlawful Pedestrian
or Conveyance Crossing
Violators are Subject to Arrest,
Fines and or Forfeiture of
US Customs and Border Protection
Please Report Suspicious Activity to
U.S. Border Patrol"
So of course they braved the possible minefield and sensor nets surrounding this sign and ALLEGEDLY traipsed back and forth across the border. They also ALLEGEDLY experimented with that strange limbo of having half their bodies in each country.
"Allegedly" is a journalistic term of art that will protect us from overzealous law enforcement agents seeking to use this instructable as evidence.
Then they went back home to the U.S., but they could have kept going into Canada carrying a cargo of books which are banned there but not here or attempted to work there illegally. They could have tried to enroll in a free Canadian University or attempted to get medical care at some free Canadian clinic and generally become a "burden on the Crown." That's the sort of thing that borders were invented to prevent.
The Canadians didn't put up their own sign though, recruiting informants and warning about how they'll punish violators.