Originally Posted by
rehoult
Officially they accept the screening from any location that meet their standards, and every airport offering direct flights to the US is obligated to screen US-bound passengers to their standards. That's why you see separate US-bound gates / terminals with their own security at airport around the world, even those without preclearance facilities (think Z gates in FRA and E gates in ZRH). However, those airport don't necessarily screen non-US bound passengers to the same standards, and therefore the US won't accept in-bound OSS as it's not done to the correct standard.
The issue on MUC-ORD-SFO (for example) is that there is no sterile immigration/customs facilities in ORD (or any other US airport), so you'd have access to your bags and therefore would need to reclear security before your connection. However, if you arrive from a preclearance facility (and therefore don't need to use the immigration facilities), you're good to go assuming you land at the right terminal or can do a sterile terminal transfer. So for example AUH-JFK on EY can sterile connect to JFK-SFO on DL. Same would go for flights from DUB, SNN, etc..
If/when the US establishes sterile clearance/transfer facilities, you'll see this issue come back to the surface. Right now from a practical standpoint they only accept screening done at a preclearance facility. They don't have to worry about if they should accept screening done in, for instance, CAI, because there isn't preclearance there, so everyone needs to be recleared anyways.
I thought about this again and think the litmus test might be an airport such as YEG. Those familiar with YEG know there's a special door that leads from the domestic concourse to US-CBP. I've never made a connection to the US at YEG so I've never gone through this door. If there's anyone out there who has made this connection at YEG, do you have to go through security screening all over again?
What makes YEG different than a place like YYZ is there's no commingling of passengers from overseas involved. The only people who can go through that special door are people who have been through security screening at a Canadian airport. For example, someone coming in from AMS to YEG would not be on that domestic concourse. This is different than YYZ T1 where inbound passengers making a Canada -- USA connection mix with other international to US passengers who just stepped off their plane.