Earlier quoted context omitted.
Why? Why are you worried about disclosing how much milk and eggs you buy? I know this is the age old "I have nothing to hide" argument but I truly am curious why people are so afraid of saying where they're travelling (train tickets) or what they're buying (shops). Don't you think that once that dystopian future comes - when that info might be used against you - we'll have much worse things to worry about?
Never thought I'd read the "What have you got to hide?" argument on HN of all places. First of all, you're being disingenious by putting up a strawman argument about "how much milk and eggs you buy." It is never about only milk and eggs. Advertising is only a part of it. Once you have an ID required to shop there, you can easily track who was there. You can follow their movements around the shop with cameras once you…
I think it's obvious that the data would not be public domain but stored strongly secured somewhere with very limited access. There are already very strict data privacy laws in Europe and the trend is clearly towards even more strictness.
I, too, don't see any issue as long the as the laws punish misuse of private data.