Why "postscript?"

I've been thinking lately about Deleuze's essay "Postscript on Control Societies," published in the book Negotiations. I'm wondering if anyone knows why the essay announces itself explicitly as a postscript.

Now, I realize that Deleuze frames the essay as a response -- or really a critical rejoinder -- to Michel Foucault's explication of the "disciplinary society" in Discipline and Punish. It may well be, therefore, that Deleuze offers the piece on control societies as a postscript to Foucault's work.

I am, however, mistrustful of that interpretation. I trace my suspicion mainly to the last few lines of the "control societies" piece. There, Deleuze states that it's the job of "young people" to "discover whose ends these [aspects of control societies] serve, just as older people discovered, with considerable difficulty, who was benefiting from disciplines" (p. 182).

It seems to me that Deleuze, rather than composing a postscript, is actually outlining a research program. This conclusion would also seem to follow from the proliferation of critical research on control, neoliberalism, governmentality, etc. So would it be more apt, then, to call the essay a "prolegomenon " on control societies? If so, then what might have been Deleuze's motivation for labeling the piece a postscript in the first place?