Originally Posted by
stargold
Who knows? Flights that are not codeshared still have blocked Y seats (e.g. LHR-NRT) as well. Besides, when it's handed over to the airport then the entire aircraft becomes available for selection anyway.
Codeshares are usually negotiated with booking class agreements, i.e. UA can sell any B class seat on ANA that is available in the GDS. It is a different type of agreement than a hard block where those seats are actually blocked for sale.
The seats that you cannot assign on NH are controlled by Japan, not out of the US.
And OP I believe you meant to say うらやましい