When you buy an e-ticket online you have to tick a box showing that you read and accept the fare's terms of conditions. (Nobody ever does, partly because it's multiple pages of upper-case gibberish, but everyone ticks the box.) That's not the Contract of Carriage, which is a lengthy document that will not fit on the back of ticket stock but is supposed to be "available for inspection" at the airport. But it's illustrative of the basis on which airline-passenger contracts proceed, e.g. implied consent.

I think it's pretty clear than neither most passengers nor most airline service people know the Contract of Carriage chapter and verse, or you'd see reroutings due to delays, etc. handled with less debate and upset. But the airline could probably argue that, as with the box-tick ploy on nonrefundable fares, making the information available somehow is all that is now "reasonable and customary".