Those folks that go to Asia from the US don't need to clear any customs in YVR. However, when they get back to US, they only need to clear the US immigration. However, almost everytime when I get back from HKG, while walking on the bridge, the Canadian customs (sometimes with dogs) will check whether you have the passorts and the visas. May be they don't want ppl who can't enter Canada to claim for refugees.

Hope someone can verify it for me