Most countries that I have visited by air (off the top of my head: UK, Ireland, Japan, China, Taiwan, Korea, Germany, France, Italy) segregate international departures from domestic departures.

The US is weird in that it has no immigration exit control: that function is handled by the carrier rather than by immigration agents. The US also has no concept of sterile transit from one international flight directly to another. Here's what I mean:

LAX-NRT-HKG (transiting in Japan): Passenger deplanes from their LAX-NRT flight through the segregated international arrivals corridor (i.e. the other side of the glass partition from the terminal). At some point before immigration and customs, there is another corridor marked "International Connections." Passenger goes down this corridor to a security checkpoint, gets screened, and walks out into the international departures area to catch their next flight. They have not legally entered Japan, so they do not go through customs or immigration, and their bags are checked straight through to their NRT-HKG flight.

NRT-LAX-GRU (transiting in the US): Passenger deplanes from their NRT-LAX flight into the international arrivals area. They go through US immigration's passport/visa check despite the fact that they do not intend to stay in the US for more than a few hours. They collect their checked bags and take their bags through a customs check. They then re-check their bags, go upstairs to the ticketing concourse, go through the TSA shoe carnival and finally catch their LAX-GRU flight.

On the other hand, the US system makes a domestic-to-international connection much easier. If you fly OKA-NRT-LAX, you have to leave the secure domestic area at NRT and go through both security and immigration exit control to get to the international departures area for your NRT-LAX flight. If you fly LAS-LAX-NRT, you never have to leave the secure area at LAX unless you are forced to change terminals, which saves some time.

JAL has "international" flights from Osaka and Nagoya to NRT for this reason: passengers clear exit control at their origin and the flight arrives at NRT as an international arrival so that passengers can use the transit channel rather than getting the run-around through international departures at NRT.