I've read several different takes on this issue, as well as experienced it myself. What I've come to find is 1) it is a pretty bogus ploy of airlines to try and get you to pay for a roundtrip ticket, and 2) every situation is a little different so it's hard to come up with a definitive answer.
I along with my roommate down here flew in from the States with one-way tickets. He had zero problems, I was told I could not board the plane until I had a return ticket, that it was the policy of the Argentine government, and there was absolutely no chance of me getting on the plane if I didn't. The ticket agent lady was borderline rude telling me about how she has been in this business for 20 years, and yadayadayada. This was also on an airline (Mexicana) that could not find our reservation in there computer.
What ended up happening is we had to call customer service and they told the crazy lady how to do her job, and lo and behold did we have to pay for a return flight? Not at all.
I've also had other friends not have a problem. Furthermore, no one that I know has ever been asked to show proof of a return flight upon arrival in Argentina.
I would try to call the airline out. Talk to their customer service department. I bet you will not have to. If you do, the crazy ticketing lady said that I could buy one and then just cancel for a refund upon arrival here.
Hope this helps.