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Health security theater catches about half of cases.

It's not security theatre when it's that effective. It provides real benefits in terms of slowing the spread of the disease.

Wouldn't the prudent step be to globally announce a two-week quarantine of all arriving passengers?

The practicalities of that make it pretty difficult. And it means that large numbers of people who don't have the disease will be kept near those who do, meaning some of them are likely to die. (As happened on the cruise ship). I don't think the morals and ethics of this are as obvious as you are making out.