Earlier quoted context omitted.
> If someone fixes or adds something valuable to GPL source code and wants to charge for it, then why not just charge for the patch or the additional code? You can legally do this, but it will not be easy. You cannot distribute a binary in this way, if you do your patch becomes GPL so the first person who buys your patch can legally give it to everyone else. That means you have to sell the source code to your patch a…
As other people already said, intellectual property laws restrict not only the work itself, but also derivative works. A patch is a derivative work, no matter how you try to dodge that part of the law. So it would still be violation of copyright.
For example, the original work might be very valuable, a significant work. It could be the collective work of many authors. But it has been made available for free. The derivative work, maybe patches or some additional source code by a single author, is also valuable but on its own much less so.
Then the large original work is packaged together with the patches and additonal source code as a "derivative work". It is commercially licensed to an end user who sees the value as a whole, most of it coming from the original work, which of course was available for free.
Depending on whether attribution is given, the user may have no idea that the core of the product was open source and available for free.
More importantly, there appears to be no assessment of the relative "value" of the free portion versus the closed-source commercial portion of the product.