Double optin option does nothing

When "Require subscribers to Double Opt-in" is ticked under Subscription Settings on the Edit screen of a form on Sign-up Forms, users are still automatically signed up when they fill out the signup form. They receive no email asking them to confirm their email address. When "Send a welcome email to new subscribers" is ticked, they receive an email telling them they have been subscribed to the list and no further action is requested.