It has nothing to do with www or http.
You are not being warned that the website is not secure - the warning is that your
connection is not secure.
There are two protocols for web addresses. One is
HTTP (HyperText Transfer Protocol) the other is HTTPS (Secure HyperText Transfer Protocol).
The only difference between the two is the HTTPS
connection is encrypted and HTTP
connection is not encrypted. The "Secure" version doesn't protect the web user or the web server where the web pages are found. HTTPS only protects the data while it is being transferred
between the web server and the user.
Most web browsers don't show whether the web address starts with HTTP or HTTPS - they display the shorthand www instead. That keeps website addresses easier to remember.
For example this page probably appears in your browser as:
"www.treasurenet.com/forums/general-discussion/592948-not-secure.html"
but in fact the full page address is:
"http://www.treasurenet.com/forums/general-discussion/592948-not-secure.html".
That little "http://" is required to make the address work but your browser hides it for convenience.
You can leave out both the http:// and the www and the web page will work as well:
"treasurenet.com/forums/general-discussion/592948-not-secure.html"
So the upshot of all this is that a "secure" web page only protects you from outsiders listening to what goes between you and the web page but does nothing to make the web page or you safer. It benefits the overall internet to have secure HTTPS connections but there is no specific benefit to you or the website.
Your browser for reasons best left to the industry to try to explain is NOT warning you that the website is not secure but only that your connection to the website is not secured. If you aren't banking you really don't have anything to worry about with a non secure connection. With a non secure connection your biggest worry would be your internet provider snooping on your web surfing - which they have been doing for years. Unless you are into something illegal even that is not much of a security concern.
There are other ways to have a secure connection without the website using HTTPS that are even more secure that HTTPS. Using a secure private proxy or Tor browser will get you a much better degree of protection than just having a secure
connection.
Bottom line is the Treasurenet website has no need of the secure version. There is nothing on Treasurenet that could make you more secure by using the HTTPS protocol. Just go enjoy the forums and stop worrying about secure or unsecured connections when you are just surfing Treasurenet.