John Gruber from Daring Fireball gives a very good description of how online ad tracking would work in the physical world:
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.)
That’s certainly something you wouldn’t tolerate. However, that’s whats going on online. In this context, Apple’s move to ask users to actively opt-in for this kind of tracking is certainly more than welcome, even if it’s delayed a couple of months. While Apple is certainly no saint and things would be different if Apple would have a different business model, no action Apple can take against the tracking industry is too strong.