It has always been like that. SAS introduced Premium Economy in early 2000s, which books in to Y and B (and S and something else.)

UA no longer codeshares with SAS but back then if you bought a full fare Y or B ticket from UA on SAS for exactly the same price then you would actually be seated in economy. I would often buy tickets with IAD-CPH in B class, flying SAS in PE, and then fly STO-FRA-IAD on the return in B where I would upgrade the UA leg with 10k miles at the time. SAS had no problem selling me this ticket and would book the UA flight number rather than codeshare so that I could upgrade without problems on UA.

I think the same thing happens (seated in economy) if you buy ull fare Y class from ANA, which will probably price the same via UA. You won't get the discounted PE fare (E class) via UA, but even if they did then you would find yourself in economy if you purchase through UA.

Thankfully I don't live in the US.