Siddles not in the side I guess cause you can obviously play only 3 quicks. Johnson will always be there, and I dont know the coach came out and said Harris is the number 1 bowler Australia have (news to me). So it only leaves 1 spot.
The reason Copeland is there is just cause its sri lanka. He will be used in the same way australia used michael kasprowitz. By the end of kasprowitz's career he really only ever played in the sub continent and copeland does the same thing (A bowler who just hits the spot, bowls cutters and just looks for seem movement - no pace at all).
I think Copeland might go ok over there, this wicket is condusive to seem - but yeah it wont last after this series.