Okay, honestly. Sometimes I feel like people don't grasp the most basic concepts.
When you're in a public bathroom that doesn't have multiple stalls, you're supposed to lock the door. That way, people don't walk in on you when you're using the bathroom.
What a novel concept!
The only way I can understand the door not being locked is if it's a child who doesn't know how to lock the door or whose parent doesn't want them to lock the door. In these cases, though, I think it's rational for the parent to go in with them or to wait right outside the door so people don't accidentally walk in on their children.
Today at Old Navy, I walked in on a mom and her two children in the bathroom. Like, what are you doing, lady? Just lock the door! It's so obviously a part of the steps you're supposed to take.