I believe the thinking behind it is that they only want someone for this one game. Shane will be training from tomorrow, playing the match on Tuesday, then leaving the squad on Wed. It's such a short time-frame, and I believe Shane is in Aus already.
From this, it looks as though North is now a serious doubt for the test, so Zebo, Cuthbert and Maitland are getting protected? Still a little odd that none of those three aren starting. There's also a lack of centres to cover on the wing, and with Hogg covering 10, they can't shift a fullback there either.
North and Bowe are likely to be fit for the second test most likely, as well as Roberts and Tuilagi, so they didn't need a full call-up to the squad, so Shane covers a single match.
I know it's all a little odd, but I don't care. It's just added a little more excitement. Everyone will just want to see if Shane still has it. Shane must be confident of his fitness and abilities, otherwise I doubt he'd have agreed. He wouldn't want to come in an play a shocker.