Richard Best has a really interesting post about how few WordPress developers understand the GPL:
Theme and plugin shops deploy a range of approaches as to how they describe the GPL-licensing of their products, from good to not-so-good to bad. I’m not suggesting that all theme and plugin shops whose terms fall in the not-so-good or bad camps are deliberately misleading customers. I suspect the reality is that some are doing this, for economic reasons, while others or their lawyers have either misunderstood the GPL or used language which just isn’t quite right, if not quite wrong.
This is true. I’ve seen multiple plugin and theme developers add so many restrictions to their “GPL” software that any freedom of use is lost.
I don’t neccessarily blame the developers for their failure to fully understand the GPL. It’s a license with multiple interpretations and loopholes.
Perhaps the best solution is a boilerplate Terms & Conditions. New developers can pick up and copy-paste the licenses for easy use.
Creative Commons makes it really easy for people to understand their licenses: https://creativecommons.org/licenses/. The GPL does not.