Earlier quoted context omitted.
They may not want you to know. They may have had a reason for not telling you about the event (i.e. they're going to a party thrown by your ex-girlfriend). There's a number of social reasons people don't generally let people know everything they do.
If they don't want things published, they shouldn't be publishing them on Facebook. Nobody should be using Facebook as their personal calendar. Also, if a detail of how Facebook handles event queries is enough to "keep you up at night", how the hell do you handle nuclear proliferation in the CIS states?
Most folks I know on facebook tend to use it to communicate with their friends. All of their friends are in one simple place, and they can arrange events easily.
They may not be aware of what they're doing until its too late or they read an article like the OP.
Its just ease of use that people have grown accustomed to.