Of course, such a treaty would severely undermine the court, which Australia has ratified. Though a body such as the ICC is looking rather unfashionable in the landscape of Bushian international relations (i.e., "I'm the sherriff, you're the posse").
It could be like most-favoured-nation status. If you play by our rules and keep profits flowing to head office unimpeded, we'll consider you a member of the society of "civilised nations" above committing war crimes, and you can get away with dealing with pesky dissidents any way you like.
Already the US signed the treaty with Israel, which presumably implies that America holds Israelis to be immune from prosecution by the ICC. Wonder when China will get to join the club.
why did east timor do it? were they about to get 'panama'ed? was it the oil? was it the guns? was it the drugs? was it about to become the new fernando poo fnord?
No idea. Presumably, when you're a small, new state that just got independence from a large, heavily militarised, atrocity-prone aggressor, you can never have too many powerful friends.
Imagine if this kind of thing blossums into a general treaty; suddenly no country will be handing over any foreign nationals for trial for war crimes. Trials like that of General Augusto Pinochet, and Nazi war criminals hiding in South America, would never go ahead again.