Imagine if you were out shopping, went into a drug store, examined a few bottles of sunscreen, but left the store without purchasing anything. And then immediately a stranger approached you with an offer for sunscreen. Such an encounter would trigger a fight or flight reaction — the needle on your innate creepometer would shoot right into the red. (Not to mention that if real-world tracking were like online tracking, you’d get the same creepy offer to buy sunscreen even if you just bought some. Tracking-based offers are both creepy, and, at times, annoyingly stupid.)
Or imagine if you found out that public billboards were taking photos of people who glance at them, logging those photos to a database, and using facial recognition to match them with photos taken at point-of-sale terminals in retail stores. That way, if, say, you were photographed looking at an ad for a soft drink, and later — hours, days, weeks — purchased that same soft drink, the billboard advertisement you glanced at hours, days, or weeks before could get “credit” for your purchase.
We wouldn’t tolerate it. But that’s basically how online ad tracking works.
As Gruber notes, the tracking industry is up in arms about Apple’s forthcoming tightening of privacy in i(Pad)OS 14 for the simple reason that it exposes their tracking for what it is: unwanted.