It's not a rule issue, it's a program/software issue. ANA also says this and despite both saying this, UA's page for seat selection is still there AND users are still allowed to assign a seat for themselves, only to be canceled after payment because of "basic economy". NH's system does not display that kind of page.

FYI, I booked on JAL via AA and I was able to assign myself a seat even in Basic. That was 2 years ago.