I have never heard of such a rule. Once one has one's checked luggage, what is in checked, carry-on or on one's person, is all the same and all are subject to search. The same is true for the written or electronic declaration. It is either true or false and has nothing to do with where one chooses to put one's extra bottle of booze.

When they first started doing this at IAH, I didn't realize they kept my GE slip at immigration. When I got to the baggage claim I thought I'd lost it and was searching my bag, my pockets and the floor around me.
But there is no "customs" like before (other than an agent at a podium off to the side barely paying attention). You just walk right out.

No, it's a minuscule missed opportunity for US airlines. There are a small number of itineraries where it makes sense to do an I-I connection in the US.

Well, the majority probably involve Canada, and they've solved that with pre-clearance.

You should avoid opening your bad until after you clear customs. They do not like you going in your bag prior to the inspection point.

I thought so too, but maybe our idea is old school. After all, I never saw a sign, but for some reason I always thought it was a No-No as well. Did it used to be?

Canada doesn't have airside transit either, at least not on US-YYZ-EU flights.

What itineraries would you want to transfer through the US?

I have heard of it, and I heard of it when I was admonished by an agent at one point before I'd heard of it.

They don't want you taking something that was in your bag and hiding it on your person, as there are items that can get to bag claim at your destination in your bag that would get caught in a bag search but they wouldn't search your person for or wouldn't find on such a search. They also don't like the idea that you might have a weapon of some sort in your checked bag you might use to "escape" customs.

Yes, I've seen people do it and no one cared, but best case scenario is no benefit and worst case you get extra attention you don't need.

HKG-YVR-JFK is I-I transfer. Just a quick security check.

Really? Can you please provide the reference as I do this at every US airport when I am connecting from an international airport. Often it is to put the duty free liquids in the checked luggage and other times it is to not need to carry the unnecessary clothing. I do not do this in secret but completely in the open. However, I am certainly not trying to violate any law. I have never seen any signs prohibiting this behavior.

I think the baggage collection area is a more secure area than the busy baggage drop area.

They had to clear security at JFK so why do it again at YVR?

You have read my post backwards. :)
Starts in HKG, then YVR, then JFK. Security stop at YVR for everyone. Customs and Immigrations at JFK.

The other way... JFK-YVR-HKG, security at JFK, business and first stay on the plane at YVR, coach goes through security only at YVR (I think). Customs and Immigrations at HKG.

LMAO! Oops...

Here's the part that I don't understand. You have to clear security at HKG to get on to a plane. So why go through security again when it lands at YVR?

Funny in the other direction, F and J stays on the plane, but Y have to go through security. Guess that's one of the perks of F and J.

Yeah, if you're going for a proper, nice I/I experience I would not pick an airport with transfer security. I don't think "quick" and "security check" ever wind up in the same sentence in Canada :)

I did ask. The FA said it is a USA requirement for everyone to clear security. I guess HK does not have the same requirement. (Seems a little silly, but after 9/11, I guess they want to reduce chances of a problem in US airspace.)

Reasons I think Y is required to leave the plane:
1. People do get off/on in YVR. In biz/first every seat has direct aisle access so anyone who needs to get off can. In Y, it could be a bit of a cluster.
2. Cleaning crew. Easy to clean around some people in First/Biz. Harder to do in Y when the window might have an open seat.