The issue I mentioned is not a matter of trusting the government. If you read the form when you sign up for many discount cards, the retailer specifies limits on how your purchase data will be shared. Releasing it to the government would be a violation of that agreement. And it's really moot anyway, as one can simply pay cash and not use a discount card, so the only people who would have data transmitted from discount card databases would be those who don't care, as you do.
Look at the current relationship between the federal government and the banking industry. Your data is transparently available to the feds under many circumstances, and court orders are easily obtained in any case.
A court order is still required, which limits the ability to obtain data in bulk - and, in addition, probable cause to obtain a court order needs to come from other sources, not the financial data itself.
Consider the library situation that has developed.
And libraries are nuking records as a result. That law of unintended consequences thang that is a rider on every bill passed by Congress but that is never considered during debate.
I'm sorry, but ideals don't seem to be in place any longer with regard to privacy from government intrusion.
I've developed an alternative view towards this problem. Gun grabbers sometimes postulate the idea of keeping guns legal but outlawing ammo. I think we should reverse that concept regarding personal data and its use by the feds - we really can't do too much about the mountains of data that are legally available from private sources (the ammo) - but we CAN institute controls on governmental databases and analytical tools that use this data (the guns). So IMO the best approach is to worry less about the minuitae of the data (where the feds can bury data-gathering mechanisms deep in legislation where only the most masochistic wonks can find them) and instead concentrate on review of the databases, which are much larger projects that sooner or later appear on the political radar screens.