> Do they analyse the prices actually paid by the average attendee, which includes prices paid to scalpers?

Unclear--the source describes the price variable as the "weighted average of seats normally available to the public" [1]. (It's also from 1992.)

> if scalpers don't cause attenders to pay more overall, then why do they exist?

Same reason underwriters do: they reduce risk for the seller and increase convenience for the buyer. You can absolutely have a situation where a minority of buyers pay more, thereby allowing the remaining 90% to pay less.

[1] https://sci-hub.ru/https://www.jstor.org/stable/2487999

The scalper is trading volatility (they may or may not sell the ticket for a higher price) for the opportunity of a return. The venue is getting the opposite side of the deal. Guaranteed income now.