Originally Posted by
beachmouse
Note that this is not US border control per se. It's because an airline can face large fines if one of their international passengers is denied entry upon arrival to the destination country. So the airlines tend to be rather to very careful about making sure all passenger paperwork is properly in order before people board an international flight.
Not only would the airline pay the big fine, the airline would be required to fly the denied person back to the US, immediately. If the return flight were sold out, the airline would be required to involuntarily bump another passenger to make room for the denied person.