In most (note the generalization) countries where carrier personnel collect passport and other document data where there are no exit controls, it is for the carrier's protection.

If, by way of example, a carrier transports you from JFK to LHR and you are turned away at LHR because you lack a passport, the carrier faces a fine and the responsibility of returning you to the US. Thus, the carrier checks to make certain that you have a passport before permitting you to board at JFK. But, this is not an exit immigration check for the US.

Nonetheless, the data may be useful for other purposes.