That's kind of true but that's only because adtech is siloed. Let's say you're identified by Facebook as having an interest in the Sony headphones. User clicks an ad and heads over to Amazon to purchase. The purchase is made and Amazon knows the conversion happened but Facebook does not.

Facebook does purchase credit card usage information but it's not nearly as perfect as having a conversion pixel on the site.

Future laws should really focus on cross-site / app tracking and the legality of purchasing third-party data.