True.

But the idea I had in mind with this question was that there is often an initial free source of high value code and then someone may add an addtional amount of their own code that is by comparison far less valuable.

The end user of the commercial product if it is closed source may not be able to see the extent of the contribution of that free portion to the value of the product as a whole.

For lack of a better hypothetical, imagine a product that is a combination of 1. GPL licensed command line executable that anyone can download and compile for no charge and 2. a GUI "skin" written by someone who has no copyright in the command line executable.

The author of the skin, let's call him "the entrepreneur", wants to charge end users.

The entrepreneur packages his skin together with the GPL licensed command line executable as a closed source commercial "product".

As you state, "Nothing stops anyone from charging for the GPL code in the first place."

But in this case, as is common among GPL-licensed programs, the author who wrote the command line executable never charged for it.

The problem I see with this situation is that because the product is closed source, the end user may not be able to determine the extent of "the entrepreneur's" work.

For example, his contribution to the "product" may be quite small in comparison to the work of the original author. That is, the core of the product, the most important bits, may comprise the freely available GPL source code, available to anyone for no charge.

This is not a situation that I expect anyone else to care about. But as an end user, if I were faced with paying for a commercial software product, I would want to know about the entrepreneur's use GPL source code that I could obtain elsewhere for free and the extent of the entrepreneur's "contribution" in relation to the original author.

Returning to my original stupid question, why does the entrepreneur not sell his skin separately from the command line executable. It is a rhetorical question. (Commenters tried to answer it anyway.)

Assuming the command line executable is a well-written, highly valuable program and an end user does not know about its existance, the entrepreneur's product may look quite valuable. The entrepreneur may be piggybacking on the value of the GPL source code but concealing this by keeping the source code secret.