Why cash is truly private and digital money isn't

Every time you make a digital payment, you leave an indelible record. When you use cash, it's totally private. With central banks all over the world considering creating digital forms of the leading currencies, what are the (mostly comic but still alarming) implications? Could Prohibition come back and be enforced every time you try and spend your dollars, pounds, or Euros on things that are bad for you? It's very possible. 

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