I noticed the same problem too. Although the normal registration form there is a field you can set.
When you use the custom registration form control, there is absolutely nothing you can set. If you look into the code of the form control "Password with confirmation" (/CMSModules/Membership/FormControls/Passwords/PasswordConfirmator.ascx), you'll notice the label field is set to display="false"
<cms:LocalizedLabel ID="lblConfirmPassword" runat="server" ResourceString="general.confirmpassword" AssociatedControlID="txtConfirmPassword" EnableViewState="false" Display="false" />
In de asxc.cs file of this control you'll see the value will never get the value true. Even worse, you can't find this label in the ascx.cs code. The textbox will be set to true, but the label won't be
public override bool Enabled
base.Enabled = value;
**txtConfirmPassword.Enabled = value;**
passStrength.Enabled = value;
Seems like a bug to me.
I reported it, but you could create your own form control based on the existing one and just add the lblConfirmPassword.enabled = value; inside the function above