Thank you for your response.  

Each terminal currently has a single exit path from the airside area (inside security) to landside (outside security). For example, in Terminal 2 someone leaving the airport passes a sign stating "Beyond this point you must be rescreened" with a security officer stationed to stop people from re-entering, then passes through a series of automatic doors that only allow one-way travel towards the landside exit.

Similarly, there is only one path of travel from landside to airside in each terminal, which is through the security screening area.  These are separate from the exits.

It's possible that your software is set up to block routing through the barrier=checkpoint nodes that represent the security screening areas by default.   

Similarly, if you are using a quality-assurance tool it's possible that the path through customs and immigration will be incorrectly flagged as a dead-end one-way, since the international flight it originates from isn't mapped in osm.