Because meanness stands at the basis of our Western civilization, of which this website is more or less a part of. Think of Cicero's "Catiline Orations", which was an ad-hominem attack through and through, Aristophanes's plays, Lucian of Samosata's works, almost everything written by Swift, Shakespeare's Marcus Antonius's speech, which is another much celebrated ad-hominem, and the list goes on and on. Adversity helps us move forward.
To clarify my earlier point, there are few things as satisfying to read as a witty evisceration of an argument or stance. For a public persona, I don't see much problem with applying it to their wider body of work as long as you think the criticism applies well overall. My earlier statement was fairly ambiguous in this respect.
The author is free to write that. But I'm not going to spread the link.