I think Australia pays more, and if I remember correctly Marcus played for Maine after his season ended.
Generally correct. The other point is that the D-league has a draft. You can't just send players there at the beginning of the year, because they might not wind up with Maine. They have to wait a few weeks until the draft is completed, sign a D-league contract, and then get assigned to Maine. So Thornton hung out in Australia until such time as he could actually play in Maine. Because he finished in Maine last season, this year he can avoid the draft and head straight to Maine if he's still unsigned by the Celtics.
It's a stupid system, and I really hope they fix it with a new CBA.