Well it was a tongue in cheek comment. Point is any illegal looking at that sign knows the probablility of being prosecuted is low.
Since the sign was only in English and the vegetation very green, I'm betting it was on the Canadian border. I have not found one on the Mexican Border. "Our" government probably would not put one there for fear of offending Mexico and Mexicans.
When "our" government doesn't acknowledge U.S. laws - but hides them from view to win the favor of certain foreigners - it is as bad as it gets.