I think it's misleading to view the matter in these terms. I have never once been asked to show my onward/return ticket at immigration in any country that requires it. The "enforcement," however, takes place at airport check-in for your flight to that country. The check-in agent can look at an Argentine citizen's reservation and see that s/he's flying home. Check. Looking at a non-Argentine citzen's reservation, the agent can see if s/he has a round-trip ticket. Check. No return/onward ricket? Possibly no boarding the flight to even get to Argentina in the first place.

That's where the problem will appear. And to say that the check-in agent never looks? Of course they look at these things.