if you are using multiple cards, etc. (They don't know it is you, until they cross reference)

• It's a big deal because legally collected, anonymized data from one or more sources can be purchased and then de-anonymized. Generally you don't get to consent to this. The de-anonymized data is personally identifiable, giving companies with money as much information as if they carried out very invasive data collection.
• It's a big deal because it costs you money. Companies want to offer you the least discount that will get you to shop. Did you know that companies offer bigger discounts to the wealthy than they do to the impoverished? That's because the wealthy have more choices. I am not a fan of corporations weaponizing my socio-economic status, or making inferrences about it from general trends in their data.
• It's a big deal because corporations make inferences about me based on very sketchy data, and tailor their behavior to who they think I am. This is maybe harmless when Wal-Mart guesses my wife is pregnant because of the last couple of items she bought. It is way more sinister when a politician's back-office infers my party affiliation, voting record, religion, etc., from something as sketchy as my zip code. If police or TSA screeners do this, we call it racial profiling or ethnic stereotyping. But if Wal-Mart's computers do it, we are expected to think nothing of it.
• It's a big deal because it's asymettric. Spending a million dollars on data analysis is nothing to a big company, but it's beyond the means of all but the 1-percenters among us citizens. Same thing with spending a million dollars on lobbying.
• It's a big deal because big corporations spend millions of dollars a year lobbying congress, planting corporate-viewpoint-friendly editorials, and sending consumers soothing letters trying to convince you that it's no big deal. If in fact it was no big deal, there would be no resistance to improved privacy legislation. But invading your privacy is lucretive for companies. They don't want to give it up.

Mike Marynowski wrote:

Why is everyone's "privacy" of such grave concern to them, in the case of anonymous usage data?

R. Giskard Reventlov wrote:

Messy, inconsiderate, lazy buggers.

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