Because Facebook is pulling a bait and switch.
The vast majority of users (debatably all, outside of the ones that signed up since this went into effect) signed up for Facebook under the pretenses they had control over their privacy. To varying degrees depending on when they signed up, of course- for instance when I signed up it was (in theory) only shown to users at my school, and I had the option to only allow friends (which of course I have control over).
I agree- for the people signing up from this point forward, they're opting into it and should educate themselves enough to know what they're opting in to (though I would argue they get duped anyway, but such is life and was, for instance, MySpace before). But what about all the current users, that aren't even going to realize that all their data is no longer private and they no longer have control over it?
