I question that. My guess is that the vast majority of the people who get a seat assignment when they book, never consider the possibility it might be taken from them. Most of the time, people are conditioned to not want to cause a scene, but I can bet no one is too happy about it when a breach of contract happens.
Airlines are unique in that lots of people go along with this. If you sign a lease to rent your house to someone and on the day they arrive with their moving truck you tell them you decided to lease it to your brother, there is a good chance you will end up in court.