I never claimed that citizenship is not the main criteria for the VWP. But just because it is does not mean that everything that feeds into the VWP has "citizenship" as a main purpose. If that were then nobody would have gone through the trouble of creating ESTA. The checks that were in place before were sufficient for that purpose. There were no complaints about that. You can't just look at things in a vacuum as if there was no world before today. Just because something on the surface has one main task does not mean it was mainly created to perform that task.
The reason ESTA exists is, as you correctly note, that the US wants to do certain checks on everybody, including VWP travellers. Before the introduction of ESTA they could not easily do that. Now they can. The reason they ask for citizenship on the same form is then done for efficiency (to check all the main eligibility criteria as well).
The fact that ESTA collects data and later allowed to easily add additional criteria for the VWP and to collect a fee, was certainly not unwelcome by the creators (but this is not why it was designed).
Why have I even started to reply in the first place? Your original comment states
- "The online ESTA form is required to determine if you're in a population that's generally eligible for visa-free entry (so that the airline and other systems know whether to require a visa for you)." (it is not required to do that as history before its introduction shows – there are alternatives to do that just as effectively)
- "It simply says 'Yes, you appear to be a citizen of a country which, in general, has visa-free entry to the US'" (no, it states much more as evidenced by the fact that countless of citizens of VWP nations get denied their ESTA application)