Mr. V, if I understand the sites correctly, it works both ways.
A person has to give their actual name and email when signing up to get the reports. The sites make additional income by making available to people a listing of those who have inquired regarding them. The sites are kind of working both sides of the fence. If Argentino knew which site KJ used, which could probably be established via a link posting, KJ's anonymity could be blown. Alternatively, KJ may have used another person to get the Argentino information (I would, if I were doing it), so violating the terms of the signed agreement could create problems for other people.
These sites are not all providing the same information, so depending on exactly what was featured in an Argentino expose would tip off the reader as to which particular site was the info source. Then one could sign up for the site and get a listing of who had inquired. The sites have quirky, idiosyncratic differences in the info listed that may expose which site was the source, basically. Then you sign up and run a reverse inquiry seeing who was trying to I-Spy you. If you know a person is male in Nevada and did the search in a particular time frame, it'd be pretty easy to tag KJ with his real name.