Simple answer? They want it to.
Think about it. There’s a line that keeps popping up regarding Facebook (and yes, it applies to Google and a bunch of others as well, but Facebook is the subject today):
If you’re not paying for it , you’re the product.
We all know that Facebook is supposed to be worth something like eleventy-billion dollars. Why? It’s not because they show lots of ads and make money from that (although I’m sure they do). It’s because they provide advertisers and marketers with the exact sets of eyeballs that they want to reach. And how do they know exactly who to include? Because we give them that information freely. If we could easily set Facebook’s privacy settings so that they couldn’t share that information out, they’d be out of business. So they make sure that to block it all out, we need to go all over their pages to ferret out every last little setting. It’s also why everything they do rolls out to everyone and then you need to opt out of it.
This most recent change to Facebook’s UI? I’d be willing to bet that it’s similarly designed to gather more information about who you are to marketers. It makes you categorize your friends to identify who you want to hear from more frequently… certainly seems to appeal to marketers. And the impending launch of Timeline? Making it feel like you need to share <i>everything</i> on Facebook? Sounds like exactly the sort of thing a marketer would want.
Lots of people worry about Google and being evil. I don’t. I worry about the massive data store Facebook is building and how to control what they do with it.