They also libel you after you have been banned because you cannot respond.
see: http://en.wikipedia.org/wiki/Cubby,_Inc._v._CompuServe_Inc.
And the subsequent Section 230 of the CDA: http://en.wikipedia.org/wiki/Section_230_of_the_Communicatio...
In no way was it implied that an ISP or other information transfer network (ie. the post), had to moderate its data transfers.
This is about the specific service that uses the network, not the network.
But you are right, if company employees are saying things specifically about people that would be considered libelous that's a problem.