Not really, airlines must comply with their contract of carriage. In this case, the UAL employees were passengers not aircrew. They have the same rights as any other passenger -- but no special rights.
In this case, under no analysis did UAL comply with their contract of carriage. Accordingly, they breached the contract and violated Federal law.
Airlines are common carriers, as such they have special privileges, rights and duties that go beyond normal private enterprises.
On its face this makes no sense. Why would UAL bump four paying passengers in favor of four non-paying passengers?
As I've speculated on this thread, United was almost certainly obligated to fly those four crew members even if they were better off keeping the paying customers happy.
I don't want to see mere speculation on this from "armchair experts," but I'd be curious to hear from people with a legal background and/or airline industry background who might be able to answer this question: What happens if an airline is faced with a situation where they have two competing "contracts" at work -- their obligations to their passengers and their obligations to their crews?