SSL encryption causes extra CPU processes to encrypt and decrypt the information before sening and after receiving data. On busy sites this can cause extra delays in page load, so not all sites force SSL on all pages. You are most likely being redirected to the non-ssl page for a page that doesn't have any private data or the site login page.
This is not a good practice for security, but it is for performance reasons. Slowly over time you will see most people forcing SSL on their websites due to recent announcements by Google. Otherwise if you do not see the lock icon on a web page then look for a login page and see if SSL is enabled there. If so then the site should be fine. Just be sure to manually log out when you do not need to be logged in to use the site.