Australia closes its borders and doesn't even allow its citizens to come back
This is not true. Australian citizens have an
automatic exemption to travel restrictions into Australia. Australia does, however, restrict its citizens' ability to leave the country.
Their border policy is very popular and is intended to stop the spread of covid into their country. Unfortunately, they have been slow at distributing the vaccine, which ultimately would give way for them to open up their borders again.
I think that many countries would like to do the same but controlling large land borders seems impossible in today's world. Having your own island makes it so much more easier. Iceland and New Zealand did the exact same thing. Even UK tried to do it during the height of the corona pandemic. As long as these measures are short term and time limited. I don't see anything wrong with it.