I'm still not entirely sure, and I don't have a quick, simple answer. There are a few reasons, however, that stand together to make a rambling explanation.
First, it's a way to prolong the moment during a reading where the audience has a chance to interact, one-on-one, with the author. I figure that the self-effacing portion of a humorous inscription request helps disarm some of the concerns that might ambush someone considering whether or not to insult a stranger in writing.
Second, these guys write stories about imaginary people for a living. I can't envision anyone outside of comedians and toll booth operators who could come up with a more biting, uncommon, or prescient zinger for a stranger. I wonder also if the way in which they insult might butt up against how they write. I'd think an insult from Joyce Carol Oates could go to some pretty dark places. Or that Salman Rushdie might have something to get off his chest. Would E.L. Doctorow have some antebellum abuse to unleash? The possibilities seem endless.
Third, it's pretty funny. To me, at least.
Last, and probably what's kept me coming back for more, I get to pretend that they're my buddies, my peers. If only for a few minutes. I grew up wanting to be the next Steinbeck, then the next Carver, then the next anyone. I did and do idolize many of these authors; I love keeping a memento of the day we met unique enough to hopefully overcome whatever memory problems I eventually develop.
Anyway, their novels are already works of art. For me, the insults are the real signatures.