Maybe I wasnt clear in my original post. What is sort of odd is not so much that you clear Uruguayan immigration on the ground in Buenos Aires, but that the two separate booths are right next to each other in a departure hall at the Buenos Aires city airport. Its like you fill out the forms, give them to the Argentine immigration officer and he doesnt even have to hand them back to you - instead he could just turn on his revolving chair and hand them to the Uruguayan immigration inspector, that is sitting right next to him.

All the other similar immigration posts Ive seen are posts that are manned only by an official of one country.