There’s certainly something about anonymity that allows for such rudeness — note Bob’s lack of linkage. It’s a lot harder to look someone in the face and tell him he’s a moron than it is to do so in disembodied, nameless text.
Of course, even masked and linkless posters leave IP address footprints, so anonymity only goes so far.
But I think there is something particular about the blog as a form that invites such responses; the author puts herself forward as sufficiently interesting subject matter (whether strictly autobiographical or memoirish or not) to warrant reading, and then — literally — opens the floor to comments. One can imagine that, if there were such a comment function built into the writings of Dave Eggers (to take a not-so-random example), similar rudeness might ensue.