FWIW, pretty much every country in the world has a policy of only allowing entry to people with visas. Where and how you acquire that visa varies, naturally, but requiring one is the default position.

Yes, plenty of countries let you turn up at the airport and obtain a visa on arrival - but if you look like you're moving in (no onward tickets, etc) you're at the very least going to have an interesting discussion, and maybe get a free flight back.

This particular situation is undeniably complex.

  > ... sends asylum seekers who tried to reach Australia by
  > boat there, regardless of age or legitimacy of claims
  > or anything else.

Obviously all or most governments would rather determine legitimacy of claims prior to people arriving in-country. There are, clearly, massive and deplorable issues with the implementation of this system, but again, the principle is similar to how most nation states operate.

I think a big part of the problem is language. Like the life/choice descriptors used by people with strong opinions on abortion in the US -- hey, who doesn't love life and choice?

In AU one side will call these people asylum seekers, or refugees - the other will call them illegal immigrants. In many cases, both appellations are technically correct, but I think in almost all cases both appellations are non-helpful.