There are several factors that I think come into play here:
a) In general you find that most carriers will upgrade their own staff into premium cabins if economy is full to accomodate other airlines staff but normally will not upgrade those other airlines staff.
b) There also the matter of dress code which may prevent carriers from upgrading. From what I have seen when travelling on our flights out of LHR the standard of dress that many ( not all ) US staff turn up to travel in is far more shall we say 'casual' that most of our own staff would consider appropriate in anything other than economy and so would not be upgraded.
c) in a similar vein I think that for most non-Asian carriers in Asia the check-in process is contracted out to agencies/other airlines and as such they have to justify to the contracting airline the additional cost to the airline carrying an upgraded staff passenger in a premium cabin, regardless of who they work for however the justification may be easier if the pax is one of the contracting airlines own staff.
d) as a general observation I think that filling from economy up rather than premium down is the norm with most carries and if you work for one of the exceptions consider yourself fortunate.

For reference over 20ish years I have been upgraded on AC to business both Internationally and domestically ( also on one occasion by CP with whom we had no agreement but took my ID 90 when AC took my bag but not me ). Never been upgraded on any US carrier even on ID50 firm tickets. Was not allowed on Air NZ out of HNL once although they had business seats left ( CO kindly uplifted us .. again at a time when we had no agreement with them. Was also one of 3 members of a airline sports team left behind in YVR when MY OWN airline refused to take us when RTB and they had 4 seats empty in First.

So basically its all a lottery .. just always take loads of back-ups!!!!