Because, for the moment, so much is being kept confidential, it's impossible to know for absolute certain, but I actually think it's quite likely that these people are actually quite 'safe' in terms of allowing immigration.
Australia currently has an ironclad policy that if you attempt to come here illegally by boat, you will never be settled in Australia under any circumstances. There are very good reasons for this policy - the main one being it's the only approach that really prevents people trying to get into the country this way - but it does create one particular dilemma - what to do with people who attempted to come here by boat, who after off shore processing, are actually assessed as genuine refugees. We cannot allow these people into Australia without weakening our policy in a way that will reopen the floodgates of attempted illegal entry. But we do have an obligation under international law, and many would argue a moral obligation to help these genuine refugees who cannot safely be returned home, to find a place of safety.
For that reason, Australia has adopted a policy in such cases, of seeking to resettle such people in a third nation that has decided itself to have accepted genuine refugees. Most first world nations choose to accept a certain number of genuine refugees - including the United States.
That is what is going on here - Australia has asked the United States to include in its already determined quota of genuine refugees it is willing to accept from overseas, some of the people currently held in detention by Australia. If you don't take them from us, you're going to take them from somewhere - you've already decided that with your quota (which may be too high in some people's opinion, but that is a separate issue).
Now, here's the thing. The people Australia is asking the US to accept have already been through Australia's very tough processing system. They should have been assessed as (1) genuine refugees, (2) not likely to be dangerous, and (3) healthy. In all honesty, if you are going to accept refugees from overseas, these are probably the best ones you are going to get. And the US still has every right to assess them as well again - so another entire layer of assessment - and accept or reject every single one of them individually.
Viewed objectively, these are likely among the safest people who will ever be admitted to the United States as refugees.
The only reason they have been rejected from coming to Australia is the fact that they made the attempt by boat, and we do not make any exceptions on that rule.
The rules apply to everyone. There are no exceptions. If you get on a boat without a visa, you will not end up in Australia.
Now, the lack of transparency that we're seeing over this means I think Americans have every right to be concerned. And America is under no obligation to take these people, so if Americans don't want to, I can understand that as well. But I just think it's important that the facts are understood.
At the moment, a lot of people seem to be assuming these people are likely to be specifically dangerous - in actual fact, it's much more likely these people are safe. And I actually think that's a large part of the reason that the Obama administration has agreed to the deal. Because if you're going to take a couple of thousand anyway, do you want people who've been screened by a first world nation over a number of years with decent police and intelligence capabilities and health standards, or people taken out of UNHCR camps in Africa, Asia, and the Middle East. And remember, you can still screen every single one of them again - and I hope would.