Why are they blaming the airline for this? Where passengers end up is dictates by which gate they disembark at (international/before Customs or domestic), and that's assigned by the airport, in this case JFK.
Edit: Reddit speculation is that they were correctly assigned to an international-capable gate, but somebody -- presumably an airline agent -- opened the wrong set of doors which routed passengers into the domestic side of JFK T7.
Edit: Reddit speculation is that they were correctly assigned to an international-capable gate, but somebody -- presumably an airline agent -- opened the wrong set of doors which routed passengers into the domestic side of JFK T7.