Why do people have their Facebook privacy settings where the public (or "non-friends") can see their stuff?

Because

a) they don’t have much to hide, and
b) they don’t have the understanding that FB default settings exposes their lives to everyone.

My reason:
When I look at other people’s pages, trying to figure out if I know them, it is difficult if I can’t actually see any of their photos, or personal information.
I figure if someone wants to see my boring crap, they are welcome to it. It isn’t like I put anything there that is highly sensitive.

What @filmfann said (and with a nod to what @zenvelo said, except I haven’t “exposed my life” so much as I have exposed selected snapshots, likes, musical and literary tastes, etc.).

It helps for others (who may be looking, sometimes after many years apart) to find us.

Some are exhibitionists about their lives. I have friends like that.

1) Failure to understand Facebook’s mechanisms – They don’t realise their stuff is public
or they don’t know how to make it private.

2) Failure to understand the implications of posting publically.

3) Full understanding of both the above and choosing to post publically anyway, for reasons of publicity/exposure/reaching a broader audience.

It drives me crazy. I keep trying to make my facebook private, but no so private a person can’t ask to be my friend, and it’s never right. I swear sometimes the setting change on their own. I unfriended my husband so I could log on as him and see what non-friends can see. It only half helps me because he is still friends of some of my friends and some of my setting are friends of friends.

@JLeslie “Friends of friends” is a very public setting. When you think about it, it’s possibly tens of thousands of people.

You can still have people send friend requests if your privacy is just set to friends.

@JLeslie: I don’t understand why you would use “friends of friends.”

I have friends of friends for who can send me friend requests.

@JLeslie: I have it where anybody can send me a friend request. I guess I mistakenly thought from what you wrote above that you meant your friends of friends can see your stuff.

I try to have only friends for everything else. I even block some friends from seeing my status. But, still sometimes I am shocked who sees my stuff. I just can’t get it just right for some reason and some of it is sort of out of our control because it depends on the other persons setting.

Like, I think if I comment on this links that travel around through Facebook everyone can see my comment no matter what my settings are.

The only place I feel completely comfortable, open, honest and candid is on Fluther, because no one ever goes there.

Simple…I decided long before the emergence of Facebook never to post anything online that would embarrass me, embarrass anyone I know or that could damage me personally or professionally.

Simply stated, I have nothing to hide on Facebook because I never post anything personal or sensitive.