> I seriously do not understand where the logic is here. I can understand restricting travel in, but why restrict travel out? In my mind there is no reason why someone LEAVING your country might be a health concern.
I think I get it: Nations can restrict the liberties of their citizens, and monitor compliance with these restrictions -- impose pandemic-related social distancing or mask usage, sanction you for murder or theft or indecent exposure, etc etc -- on their own soil. Abroad, they cannot -- at least nowhere near as easily -- monitor you. So while they can restrict your liberties in the country, one liberty most nations find it very hard to deny their citizens is entry into the country. After all, WTF is the worth, the meaning, of being "a citizen" of a country if it won't even let you in? (cf Wolf Biermann.)
Put those two together, and the logic becomes obvious: If you let people out, you gotta let them back in. But you can't know where they've been and what they've done and with whom, so you have no idea whether they're infected or not. So you'll have to i) take their word for it; ii) investigate, at great effort, cost, and risk of inaccuracy; iii) put them all in quarantine, with the inevitable screeching about liberties infringed upon... Or just not let them out in the first place, avoiding the problem of having to let them back in altogether.
I'd even say their reasoning (as I interpret it) is legally, on the balance, somewhat sound: Once one of your citizens shows up at the border, at least metaphorically still outside it (although in Australia's case in practice of course on their soil already), they're only asking one thing: To be let in. And precisely because that's just one simple ask, and such a basic civic right, it's damn hard to deny them that. Within the country, though, within its jurisdiction, the state already is generally accepted to be within its rights to (sometimes drastically) limit the liberties of the citizenry. Like, hey, during this pandemic many countries have at times forbidden their citizens to leave their homes. So what's all that much worse about temporarily forbidding them to leave the country?