I'm pretty sure that, for example, the list of grocery brands someone buys using a store loyalty card isn't "metadata", and while "stealing" is hyperbole, I'm pretty sure most people would be upset upon realizing how far and wide that information is being sold.
I think even if you have no ownership of the data and stealing is not involved, that does not give the collecting or managing party the right to sell or publicize or share that data, necessarily.
There have been studies about the value (and impact) of inferences from metadata e.g. https://www.pnas.org/content/113/20/5536.short .
((Edit) Agreeing with you, steal is incorrect term in a lot of cases, however I am not sure if we can say it is not applicable in general.)
I am not sure we have an adequate legal model now to deal with it. We should probably get to developing one real soon. But roping in emotional terms from the different field - like calling it "stealing" or "robbery" or "piracy" or "stampeding cattle through Vatican" is not very helpful. It makes it look as if it's simple - if it's stealing, stealing is already banned, just use the same laws here - but it's not and those laws won't work. Real work is needed here, not wordplay.
A shopping list is metadata and the issue here is data ownership, you can't own a shopping list, you can't even copyright one, if you don't want it to be associated with you, you should be able to opt out, but no one should be burned at the stake for it.
Why should it be "opted in" automatically in the first place?