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Because the passenger has a contract with the airline and is not obviously directly bound by an international agreement like the ICAO convention. Furthermore, the 'transportation costs' mentioned in the ICAO convention are usually just a tiny part of the actual costs incurred for the airline, which they will or may try to get back from the passenger (potentially depending on national interpretation of the wording).
I can't find a link to the verdict right now, but in German jurisprudence, courts have acknowledged, that the 'transportation costs' referred to in the ICAO convention only cover the difference in direct costs of the flight compared when flying with and without the deported passenger. That means, that if the airline was only covered by their rights in the ICAO convention without having additional means covered by their T&Cs or additional agreements like these indemnity forms:
They could not demand that the passenger pays for a new ticket for the deporting flight, since the last-minute ticket price in most cases will be much higher than what it actually costs the airline to transport the passenger.
They could not demand reimburs*m*nt of costs related to potential fines or the detention of a passenger, since these are not 'transportation costs'.
The extent of the 'transportation costs' was at least discussed in one German court case between Lufthansa and a passenger, where the passenger refused to pay for the fine Lufthansa received for bringing a non-admissible passenger. The court agreed upon the passenger's argumentation that the fine reimburs*m*nt was not covered by the wording in the ICAO convention, but it was covered by Lufthansa's T&Cs and with the passage from the T&Cs being legally sound, the court ruled in favour of the airline.
Upvote:-2
I'll summarize the answers the question received in the comment section:
Comments:
- Probably to warn the passenger and a documentation to show in case of disputes. Explicitly signing such a documents will cause less problems for airlines, I think. Anish Sheela 9 hours ago
- From the airline's POV: simplicity. An in-hand document signed by the passenger will ease explanation and facilitate claims processes, as compared to the greater complexity of explaining the Chicago Convention on International Aviation, which gave rise to ICAO's treaty-like obligations. DavidSupportsMonica 9 hours ago
- Related Law question: law.stackexchange.com/questions/55700/… JonathanReez♦ 7 hours a