Just to clarify the legal position an international, or domestic, airside terminal (in Europe) is still a public place albeit with some specific legislation. However, the restrictive movement of individuals can only be enforced by certain bodies, this is normally immigration, customs and police. Security and airline staff have no such authority in public places. For example, if you refuse to adhere to security procedures at a check then the staff member can refuse you entry to the airside area but he must call a police officer to enforce that action should you insist on access.
The point raised by the original poster just illustrates an airport which is badly run and I'm not sure why he's being criticised by various people who work for airlines. All the guy wanted to do is leave an airport, hardly an unreasonable or unusual request.
There are various conventions enshrined in the human rights act (relevant only in Europe) which guarantee freedom of movement and, certainly, these are relevant in an airport but you can't really claim any infringement of those rights unless they were denied by a person in authority. An airline or security person (except in some European countries where security is carried out by police officers) is not such a person.