From my experience , onward travel is generally requested by the airlines. If they check in a passenger who does not have proof of onward travel plans , the airline may be fined. Therefore they have a fiscal responsibility for checking this. Generally immigrations does not check. I have travelled extensively in the Caribbean and there it is more of a concern for immigrations officials , as they have strict laws regarding employment by foreigners. I always traveled with a full fare / refundable return ticket.
As for Argentina specifically , wait till next week , I am sure they will change the law , or enforce one that is already on the books but has been ignored.