You know, you may be right - in which case my statement doesn't make sense. I almost never fly transborder - 99% TPAC or TATL so it's a situation I've never been in, but analysing my own question, if you land from a NRT-YVR flight, there is no arrivals lounge, you transfer to domestic and there is necessary a shower.

I wonder, though, what if you were flying, say, YYJ-YVR-YYZ - still same rules even though domestic only? I have a vague recollection of being denied showers in YVR or YYZ 5-7 years ago on a non-international flight.