Facebook is just gathering as much information as they can so they can give it to advertisers. I don't quite understand why people have to share some of the stuff they do on facebook and find that far creepier.
People might also have an unreasonable expectation that there's some law against misusing their data. Partly, it's because that is true in some areas, e.g. the phone company can't run speech-to-text on all your conversations and sell histograms of your word usage, so people might assume Facebook can't either.