They do make it clear you're sharing with a wider audience. When you click the share button, G+ tells you: This post was originally shared with a limited aud

They do make it clear you're sharing with a wider audience. When you click the share button, G+ tells you: This post was originally shared with a limited audience – remember to be thoughtful about who you share it with. This is a social problem, not a technical problem. You can't stop bits being copied, but you can make people think twice before copying.