Computers are powerful enough though, and people clever enough, that everyone can now have a Sherlock Holmes in their pocket. And large corporations can have smarts far beyond Sherlock Holmes. So what once was a "eh, there's no rule against it, if you have the information the smart guy should be able to use it", is now a case of "oh dang, hmmm maybe civilization is built on the idea that not everyone is Sherlock Holmes."
Maybe this should have been my own blog post somewhere, but anyway, it's one of the many new conundrums of our time.
Enter today, when nearly everyone can be and is surveiled with detail cops of previous generation could only dream of, and we're still dealing with those same laws, but we removed the unwritten premises that they're based on: surveillance couldn't scale.
When a massive object passes between a distant star and an Earth-based observer the light coming from the star gets deflected and focused by the gravity of the massive object. The star seemingly brightens as a result. This is called gravitational microlensing. The duration and how the brightening happens allowed scientists to determine that it was likely a Mars-sized planet and that it likely had no star within 8 astronomical units. It's likely a rogue planet of roughly Earth size.
Think about how little actual information these astronomers had. Yet they were able to make a very credible prediction on what happened. You see this all over in science, particularly in physics, where the truth is coaxed out of very little direct data. This makes me think that similar things can probably be done with data about people. This would mean that effectively anonymizing people's data is very hard or maybe even impossible.