Is your question why privacy is important, or are you thinking of something else?
Once I realized that, it was hard to think of reasons why their actions are wrong.
I disagree that the threat of outrage is sufficient to stop the information being used badly. For example, there is outrage against Equifax, but their cache of information has still been compromised. We have seen data leak after data leak from various companies. If a country's spy agencies want to go after the data, they have a lot of resources, from hacking to legislation to physical intrusion or coercion.
Add to that, it seems like Facebook doesn't institutionally care about privacy (probably because it is hard to explain something to someone whose paycheck depends on them not understanding it). For example, http://actualfacebookgraphsearches.tumblr.com has some very damaging graph searches. Or people who have been outed as gay by incomprehensible privacy settings.
Facebook is a sieve, and the reason to care about them having a lot of information is the same reason to care about privacy in general.
Actually, only the first two things happened.
https://mobile.nytimes.com/2012/02/19/magazine/shopping-habi...
It is possible that their source was lying or mistaken, of course.
If we ever come to widespread outrage, it will be much to late. In fact, I don’t think there can be outrage. FB would detect it and make it disappear. Somehow.
lol.