Have you asked them?
It cannot be a Government Requirement as not everyone has internet access ~ unless they only ask those who book on line to "register" ~ and that's pretty pointless since you must have already provided them with name, address, credit card details, email etc to have booked the flight and will need to show them a passport and credit card before you board. Tell them to p*ss off and see what happens.
My money is that it will be another details gathering scam for future marketing purposes.