The doctrine of common carriage developed with stagecoaches and trains and was later applied to communications media such as the telegraph and telephone.
A "common carrier" has a legislatively-granted monopoly over a particular route, region or type of communications. In return, the carrier must carry everything and has no right to reject particular passengers or communications.
Congress made Western Union a common carrier, for example, when it refused to carry cables from reporters to their newspapers, because they competed with its own news service.
It seems obvious that services such as Netcom and PSI, which sell only a connection to the Internet, should be treated as common carriers, while services such as Compuserve and AOL, which sell content of their own as well, are publishers of their own content and distributors of other people's speech. Thus, while Compuserve and AOL should have a right to edit or refuse to carry speech they do not like, ISP's should have no more right to do so than Western Union or the phone company.