I was reading through this discussion and I noticed that nobody gave the clear answer why some airports segregate departing and arriving passengers…
I have worked in Zurich airport as a part time ground staff, and the reason why airport segregates passengers is the security.
In the US, domestic arrival passengers share the same area with departing passengers. In fact, domestic flight passengers in most countries are not segregated. Also, in the Schengen area, passengers arriving from airports located in Schengen area also share the same area with departing passengers.
This is allowed because the security level throughout the US or the Schengen area is assumed to be the same. At least in the same country, the airport security regulations are the same. After going through the security, there is no way for passengers to have an access to other things outside the secured zone until they claim their baggage at their point of destination. Also, domestic passengers don’t have to go through passport control and customs. Therefore, it is okay to let departing and arriving passengers to mingle around in the same area.
When it comes to international flights, things get different. Even though it seems like every country in the world have the same airport security regulations, they don’t. Security check in the United States is much more strict compared to other countries. Therefore, airports assume that international arriving passengers are unsecured. If you are changing your flight from international to international, or international or domestic, in many airports you need to go through the security at the airport again since the government law of most countries forces passengers arriving outside of their country to go through security with their own security standards.
Because international arrival passengers are assumed to be unsecured, most countries don’t allow them to mingle with departing passengers. Even in Copenhagen airport, which the developer mentioned, if you are arriving there from a non-Schengen area, you will be segregated from departing passengers.