I look to the US who require immigration and ESTA clearance even for passengers immediately connecting onto an international flight… why?
Something to do with being World police.? The only country I remember going thru transit immigration checks was China on a Japan to Thailand itinerary, but I did not need a visa. International terminals are not separated from domestic and USA banned TWOV "transit without visa" sometime in the post 9/11 era . Remember we used to have gate agents walk people between flights/gates. But there would have been nothing to stop somebody from running away. Don't know if that happened much. US airlines lose out on of intercontinental traffic business because of this "security enhancement"