OK, I'll weigh in on this, simply my opinion, observations and suppositions.
In the past, between 2006 and ~6 months ago, when I used a credit card at Jumbo or Disco, and went over $1000 (pesos), the cashier told me that they had to split the bill, which was a bit of a hassle, simple because it was an extra step, signing twice, etc. I thought, darn you Disco, what a pain in the but. I now believe that this "law/regulation" (that required grocery stores to report purchases over $1000 to AFIP) has been in place for some time, so I suspect that the Jumbos and Discos were actually providing a service to their customers and splitting the bill for them so they would not have to report the purchase to AFIP.
I think recently AFIP said, "hey, we are wise to this, no more splitting the purchase (we want to know not only how much, but what items everyone is purchasing)." So Jumbo/Disco management told their cashiers, no more splitting the bill.
Yesterday I was in Jumbo (prepared to pay cash) and knew that the cartfull (actually of course only partially full) was going to be close to $1000. The cashier, saw the cart and said, "looks like you are going to be over $1000 so I will need your DNI or you will need to finish the purchase at another register" (the wife was not with me to make it easy to split). I said "but I am paying cash" and she said "sorry, it doesn't matter", to which I responded with a smile (not being in any way chauvinist but she was a cute young female) and said, "yeah, I know it is going to be really close because I am aware of the issue", and she said, "paying with cash", "yup", "well OK". The final bill was $1056. I paid and thanked her.
My initial reaction was one of WTF, I don't want this government or any government tracking my purchases (alcohol, fatty foods, etc.) that could of course be shared with insurance companies etc. Unfortunately, I am so accustomed to giving my DNI, that I did not think to say, "I don't have a DNI, what happens then"? Next time, I think I will say that.
One clarification, I am fully aware that the technology and methods for tracking supermarket purchases (when swiping a debit/credit card) has existed for many years in the U.S. at least. In the early 2000s I think, I recall there was some public debate over the use of this information and I believe that the outcome was that the information could not be passed on to third parties like insurance companies and perhaps only to the IRS (Internal Revenue Service that is the "equivalent" to AFIP) under a subpoena and/or audit purposes. In general, I believe that; and I think the American people trust that the info is not shared.
Herein lies a fundamental difference between Argentina and the U.S. There is a degree of good faith/the honor system in the U.S., pay your taxes because some day you might get audited, here, pay your taxes because AFIP is watching your every move a priori!!
It is a bit more than a little disturbing.