I flew in 2 weeks ago on the 2nd leg of a RT flight originating out of BA. I was flying Continental but this particular part was with United out of LaGuardia (to DC to BA).
The guy at the counter was training someone so when my info popped up he asked "if i had a visa?" I tried to bluff that the sticker they put in your passport for the $140 fee was a visa. He did not recognize it, but realized it was not a visa. I just made a face like "oh really, my mistake." And played dumb.
Then this next part is the crucial info: He then explained to the other guy but not secretly or anything: "It [proof of onward travel] is not required but we have to ask and make a note (in the computer) that we did. It says here (on the computer) that it can be purchased on arrival in Argentina."
So, my sense is that it is to cover the airlines ass so that if you are sent back they can probably try and charge you for a ticket back.
This was one guy at the United counter, and as with any of these large bureaucracy you rarely seem to get the same answer twice. But I know someone mentioned in another thread that there is an "official" governmental database for airlines that says what is "governmentally" required. Of course airlines are probably allowed to set any standard above (but not below) the government standard.
When I arrived in BA, absolutely no questions asked.