Yup. In the case of people like Stonebraker -- or Butler Lampson, who's also officially adjunct at MIT -- Think of it as: You get an office, a community, resources with which to do research (and a framework within which to ask for grant/funding if you want), opportunities to advise students, the ability to teach classes when you want. In exchange, you don't get much money -- but also almost zero responsibilities unless you choose to assume them.
(source: I'm a CS professor and I got my Ph.D. at MIT while both Stonebraker and Lampson were there.)
He's cashed out a number of successful businesses, so it's not likely that he needs money.
He's in his 70s. I would guess he still likes to teach and do research, but maybe not as a full-time professor.
Adjuncts don't have to go to committee meetings. :-)