Not surprising that a search yielded some confusing results-- it's a confusing topic, historically.

Without searching myself, I think one or 2 of these things come into play:

1- seating assignments on codeshares almost invariably more difficult than prime flights. This one is for sure.

2- this is particularly true with AA/JL codeshares

3. Something about -N- fares (and/or other cheap economy fares) not allowing same, if applicable

4. something about economy seating assignments only allowed within xx days (60?)

Take a look at the most recent posts in this thread:
http://www.flyertalk.com/forum/japan...deshare-3.html