Is the data the issue? Or is it ongoing interference with the algorithm that pushes content to users that's the real issue? Tweaking what the users see affects those users far more than just selling their data to make money.
[EDIT] That is, things like credit card companies selling your purchase history. Obviously they do need to retain some of that information to operate, but they shouldn't be able to sell access to it or use it for any other purpose. Ditto banks, magazines selling subscriber lists, all that crap. Should not be legal.