Google.com redirects the browser from HTTP to HTTPS if there is any other Google based service that is logged in by the user.
This happens because Google recognizes, from a cookie, that this is a Google user and wants to have the user go via HTTPS to secure the private information about this user.
The browser was reporting page cannot be displayed when we try to go to http://www.google.com
because it was actually redirected to an HTTPS page ( you can see that in the browser address bar). if HTTPS is not handled or allowed it would seem to the end user that the proxy is denying HTTP traffic for an allowed site.
A good way to validate is to find the 302 redirects in Wireshark on the client side. Also, try another HTTP based site and web browsing should work.
In order to avoid this problem you have to logout the user from the Google application. Another possible solution is to suppress the cookie, but this might affect functionality.