Because with all transactions recorded it would become possible and when something becomes possible, oftentimes govts are tempted. As an exemple, where I live the govt cross-checks your income tax filing with the automobile registration records and if there is, in their opinion, a discrepancy, you have to explain yourself.
Also my comment does not single out Sweden, it is more of a general observation.
So in a few years, the govt. will get a report at the end of the year showing that model Swede citizen has bought more than the recommended amount of meat or alcohol or, heavens forbid, has bought tobacco products. And not enough fruit/veggies. So the friendly Swede govt. will "recommend" that model Swede citizen fork over an insane amount of money and "optionally" attend re-education classes in healthy living. Can you spell Big, Big, Big Brother?
And why the fuck should this be on Hacker News??