A charitable interpretation is that this law is aimed at Australian residents traveling to other countries for pleasure or non-essential business, contracting covid, flying back home to Australia, and spreading it to others. Over the past year, Australia's covid rates have been far lower than other countries, hence why international transmission is a far greater risk.

One alternative is preventing citizens who've traveled to other countries from returning home to Australia. Presumably this isn't legally/politically feasible because it violates a citizen's right to return home. Hence the government's decision to restrict people's ability to leave instead.

I would hope tat getting an exemption is as simple and easy as "here's my lease and paystub in {COUNTRY}", but it remains to be seen how smooth and painless the process is.