Comment by krageon

The trouble is that they might target a group that you are logged to be a part of. For a concrete example, the reason the Nazis were so successful in finding Jews in the Netherlands is because the government there kept (and keeps, if I'm not mistaken) a list of people and which faith they belong to. That list was then handed over to the invaders, who made really good use of it in the time after.

Nobody before that point really stopped to consider whether or not it was a good idea this data existed in the hands of the people that had it. If they did, they probably thought there was nothing to hide (or even that it was a good idea, perhaps). At this point, we should know better. For the ones targeted, there were no bigger problems than the fact that someone had this data about them.