From wikipedia: "Stallman then enrolled as a graduate student in physics at MIT, but abandoned his graduate studies while remaining a programmer at the MIT AI Laboratory. Stallman abandoned his pursuit of a doctorate in physics in favor of programming."
perhaps he has an honorary doctorate?
1996: Honorary doctorate from Sweden's Royal Institute of Technology [85]
2001: Honorary doctorate, from the University of Glasgow [89]
2003: Honorary doctorate, from the Vrije Universiteit Brussel [91]
2004: Honorary doctorate, from the Universidad Nacional de Salta. [92]
2007: Honorary doctorate, from the Universidad de Los Angeles de Chimbote.
2007: Honorary doctorate, from the University of Pavia [93]
2008: Honorary doctorate from the Universidad Nacional de Trujillo, in Peru
2009: Honorary doctorate, from Lakehead University [94][95]
RTFA :)
Maybe universities need a new class of degree called "honorary doctorate and we mean it for this guy".
(edit: and Bob Barker, Tim Allen... obviously, RMS has accomplished more than many PhDs, but you shouldn't refer to a holder of an honorary doctorate as "Dr.")