The history behind the design
Freedom is never free, and Magna Carta proves it: the charter was forced from King John in 1215, then fought over in the First Barons' War.
Magna Carta was not handed down freely. In May 1215 a group of barons renounced their allegiance to King John and took control of London, and only then did the king agree to terms at Runnymede on 15 June 1215. Within months the agreement collapsed into the First Barons’ War.
The line “Freedom Is Not Free” is best known from the Korean War Veterans Memorial in Washington, D.C. (1995). This tee carries the same idea: rights that people take for granted were won at a cost.