Originally Posted by terrier

CBP preclearance locations do have one practical difference for US citizens: onward travel to the US is de-facto at the discretion of CBP.

A US citizen at other points of entry may be detained for secondary screening, and may have his or her belongings searched or seized, but ultimately they cannot be refused entry to the US. At a preclearance checkpoint, CBP may not be able to technically deny entry to US citizens, but they can detain them long enough to make them miss the day's flights. Lather, rinse, repeat.

This is not hypothetical; I was relatively recently coerced this way at DUB preclearance (i.e. directly threatened with being sent to secondary with the clear implication that I would not be allowed to fly) to answer questions about my employment and whether I or other members of my family hold any other nationalities - questions that I would not have to answer at a stateside checkpoint. I did not have the luxury at the time of taking a principled stand, but neither do I plan to return to the US via DUB on future trips.