When you enable SSL for your store, Zen Cart automatically uses your HTTPS_SERVER (e.g. https://www.yourstore.com) for pages associated with creating accounts, logging in, checkout (from start to finish) and your customers' account-related information. It doesn't use SSL to secure your various product listing pages or any of the "general" information pages (e.g. contact_us and privacy) because there's nothing to secure on those pages.