There is one thing about ensuring that travellers have the correct visa's and passports, quite another to enforce a non-legal requirement on travellers, the majority of whom will not be getting deported. In reality, how many people, out of the thousands that arrive each day in Buenos Aires via International Air Travel, actually get deported at entry, and are returned to where they have come from. I would suggest very few at all.
There is no legal requirement for entry into Argentina for a traveller to have onward travel arrangements in place, and in any case, it is not for an International Airline to act as an arm of any countries immigration authority, aside from a duty to ensure the traveller has a passport to board the flight, but in truth, thats for the departing countries emigration policy, again, not the airlines.
So, once again, the airlines are NOT enfocing any rules, but their own. And in many cases, simply to ensure that they can ensure they get the return flight from a traveller, hence increasing their income strain. As I say, I would love to see the statistics as to how many arrivees in Argentina are actually refused entry and deported from an International Airline Arrival.