He was questioned in Dublin at US customs, primarily because he did not have a return ticket.
The advantage of pre-clearance in Dublin is that US immigration can deny you without actually formally denying you.
The guy had no ticket leaving the North American continent. Thats a clear case of not granting access to the US via the Visa Waiver Program.
Actually, the airline should have denied him issueing boarding cards without an onward ticket. He should not have gotten to the US immigration guys in the first place.