Originally Posted by
lax01
Because you have to because the DHS insists on it.
The text on that page you quote says -
As an arrival from FCO
Barkinpark can go via the connections route to departures and enter the pre clearance area when their flight is called to do so. The initial security clearance will have happened in FCO so they still have to do the 2nd, DHS mandated, set at DUB.