It may have seemed a bit of a farcical ending to proceedings today, but the umpires knew what they were doing imo.
With the conclusion of the result not going Australia's way in this match, I am sure the umpires offered the bad light to the batsmen as a token gesture, as there was only going to be one result.
Law 16.1.6 PDF Document
It is not quite the same as written, but that is my interpretation of what happened, maybe other will interpret it differently ?
I am reading between the lines of course.