Someone can provide you with a clickable link, as in for instance this submission, and you would never even know that the content you are accessing is supposed to be "protected".
If you are going to nitpick, I will say that this is a feature that relies on browser-support. It's not fundamental to the web. Query-strings however by definition needs to be supported on the server-side. They are a part of the web. They are required for the web to work.
Why is "browser-support" relevant? Your example is not supported in MSIE. I also thought it was removed from Chrome (in the name of "simplicity"), but I may be wrong.
A link with query-strings is guaranteed to work for everyone.
http://username:password@members.example.com
I wouldn't say that means the account in question is unprotected.