A longer, and quite interesting explanation, is published here. A short excerpt from that text reads:
The origin of the “human rights in the subways” project was [Françoise Schein’s] first urban map entitled Subway Map Floating on a NY Sidewalk completed in 1985. The relationship between underground trains and the rights of man is explained on the INSCRIRE website: “For her [Schein], the subway appeared to be the most democratic place into which engrave philosophical concepts to address to the people”. Subway Map was awarded the New York City Design Commission’s Award for Excellence in Design, although the project is rather ungraciously listed under street furniture on the Commission’s website.