How sure are you? If it was the same with Twitter, the owners would be in jail - they must get around it somehow.
The law about whether the owner/editor/publisher of a social media site is liable in a potential defamation case is complex and fast moving, as a quick Google search would show you.
NSC has long taken the view that we'd rather not be a test case, having neither the time, money or inclination to defend a defamation suit brought by a claimant. Better safe than sorry, is the approach we're adopting.
As an aside, there are plenty of cases where claims have been made against Google, FB, Twitter, etc. Many have been thrown out; some have succeeded. We're not planning on being cutesy with E&W libel law and finding out where the boundaries are.
TL;DR: We're not interested in being 'sure' of NSC's ability to defend ourselves against a defamation case; we're instead interested in never having to find out.
Here's just one example exemplifying the complexities:
http://www.bwbllp.com/file/bwb-pandr-aut13-socialmedia-pdf