The outbound passenger who has cleared Cust/Immig and is in effect a US domestic passenger from this point on. The problem is the inbound passenger has left the USA and is now an International passenger who must clear some Cust/Immig point before going back into the US.

When in doubt on this think of any "Duty-Free" purchases made. If a passenger went LGA-YYZ and filled up on duty free, could they then connect to YYZ-ORD without declaring somewhere?

Shareholder:
Your example of inbound/outbound crossing paths is one of the big re-designs that these older terminals have to fix. It is not just a customs problem, as there are calls from some security circles that this has to be stopped for ALL passengers including Domestic connections. They want to force connecting passengers to re-clear security at every connecting point.