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Because in one case the target is the individual. The other case your data gets aggregated and sold as bulk in the form of ad impressions (tracking).

A better comparison would be, the gov uses your census data to mail you flyers.

YEah I stalk a TON of people so it's fine, it's just aggregate stalking.

No, there is plenty of stalking/tracking which targets individuals. It's called microtargeting.