Wants to be a martyr for free speech?

I see that the Catallaxy blog is hosting in comments what is surely defamation about a politician.  Not just over the top criticism, as is routine there, but a quite specific, and (I would reckon) perfectly actionable claim.

Why does Sinclair Davidson let himself continually run the risk of such action?   Or am I missing something about blog owners not being liable for defamation in comments if they don't notice it?

Update:  comment has been edited, and defamatory part removed.

I think the Professor should be paying me a spotters fee every time I save him money!