Many OZ travel agents don't assign seats - maybe because with QF you generally can't get seat pre-assignments and so they don't realise this is the norm with UA (and others?).

My first experience with this was a couple of years back (before I made Premier) when I arrived in SFO from SYD to find that my DEN flight was overbooked and I had no seat assignment. I got bumped and was handsomely rewarded (cash and an upgrade). :-)

Being concerned that I couldn't afford to be bumped when flying DEN-SFO to connect with the SYD flight I got them to check seating for that and found it too was full in coach. So they gave me another upgrade to C :-)

Since then I've been premier or premier exec and so always ring through to UA to get my seat assignment. Even though the TA can do a seat assignment they can't (in my experience) access premier seating, so it's a waste of time getting them to do it.