Seems like delta.com also blocks other carriers (not just Asian carriers) on multi-segment trips that include a Delta-operated leg.
For example, LAX-SYD is available in J with miles on VA, but SEA-LAX-SYD is not (Delta.com pushes you to book their own metal).
However, last year I was able to book SEA-(DL)-LAX-(VA)-MEL-(VA)-CNS with no issue (priced at low level). Seems not to be the case anymore.
UGH!