Comment 15 for bug 490380

Well, they can write to the directory. The thing they cannot do is write to the file that was created through the share (though they can do that by accessing the share locally through SMB !).

I don't think that's confusing, it's a decent default security trade-off, and is easily toggled off.
The "accept guests" is mostly there to allow read-only shares accessible to guests, so it shouldn't be removed.