The payroll is not the reason. Libraries and newsstands and post service are free from that liability either, but they don't have writers on their payroll. Newspapers that publish readers' letters normally don't pay for that. What makes them publishers is that newspapers choose what to publish. According to the court decision of 1995 [1]:

"By actively utilizing technology and manpower to delete notes from its computer bulletin boards on the basis of offensiveness and "bad taste", for example, PRODIGY is clearly making decisions as to content, and such decisions constitute editorial control. (Id.) That such control is not complete and is enforced both as early as the notes arrive and as late as a complaint is made, does not minimize or eviscerate the simple fact that PRODIGY has uniquely arrogated to itself the role of determining what is proper for its members to post and read on its bulletin boards. Based on the foregoing, this Court is compelled to conclude that for the purposes of Plaintiffs' claims in the action, PRODIGY is a publisher rather than a distributor."

[1] http://www.tomwbell.com/NetLaw/Ch04/Stratton.html

Section 230 was written after that court decision. Yet for the law to be consistent its provisions must agree with this reasoning.