Normally when you fly to the US from other countries there's extensive checks of your visa by airline personnel. However Preclearance airports are somewhat special because there's CBP agents right there in the terminal and in theory the airline shouldn't care in the slightest if your visa is valid or not, as that's CBP's job to verify.

In practice, does this mean that airlines don't validate your visa when flying from Preclearance airports in Canada/Ireland/UAE? Or do they still send your APIS data to CBP and would refuse you entry into the Preclearance area if they receive a "Do Not Board" message?