Originally Posted by GUWonder

They do have discretion, which is why some carriers' reps end up denying travel even to people who have prima facile evidence of being admissible at the ticketed destination. Not familiar with some carriers denying air travel from the US to Canada when the passenger only has her/his NEXUS for presentation? Meanwhile the same travel docs are accepted by other carriers and/or other carrier reps for the very same city pairs.

The "major fines" are not all that major for the US-Canada flying airlines; and very often the fines are not even charged and collected even when the airlines have transported an inadmissible person.