Because most LWN readers apparently believe that the people who write code (including the people who pay others to write code), have the right to decide what do do with the code.
The fact that many people are willing to release the code under copyleft licenses is something that we appreciate, but we don't believe that anyone should be forced to release code (unless it's a derivative work of code where the license requires release)
This is one of the places where Free Software and Open Source Software differ.
Free Software believes that writing non-Free software is unethical.
Open Source Software believes that releasing the code is a choice, and frequently a very good choice the from purely pragmatic viewpoint.
In the case of the LWN codebase, I would rather have more articles and no code release rather than having them concentrate on the code audit and release and go under because they aren't producing articles.