Good question. I have often wondered this myself. This is why I never try to use different airlines when connecting. I am thinking that since there is a record of your travel, the 1st airline would take responsibility if miss your connection.

I also wonder how missed flights should be handled due to transit on public or commercial buses and trains? If there is an hour delay in transit to the airport, and a person misses their departing flight, does the transit authority/company have any obligation to handle alternate flight arrangements? Once, the 1st local bus of the day did not show. The next bus was scheduled 45 minutes later. I missed my check-in deadline due to the unscheduled cancellation of my bus. Even if I called, I doubt the transit authority would be willing to do anything.