Why GPLs page such as TestWP could exist without getting trouble? On this page, you will understand how WordPress Plugins – Themes’ license work. This is clear explanation from AlienWP:
“If you don’t know already, WordPress is a software licensed under the open-source GPL license. This license governs what you can and can’t do with WordPress. While the GPL license does allow you to reuse, modify and redistribute WordPress for personal as well as commercial purposes, it does require you to release your derivative work under the same license.
All WordPress plugins and themes are also licensed under the same license. That means you are free to modify, reuse and even redistribute all the free and premium WordPress plugins you buy. The only limitation is that you will have to release your derivative work under the same license (GPL)
While modifying and redistributing a plugin/theme developed by someone else is not illegal, but it can be considered unethical by some people (especially developers) in the WordPress community. And because the license allows you to modify and redistribute other developer’s work under your own name, there are a lot of people who are taking advantage of this license.
These stores are also known as GPL Clubs in the WordPress community. The reason why these stores exist is that people can’t always afford some of the more expensive WordPress themes and plugins fees, especially if they don’t know that the product will be exactly what they need. These stores don’t develop or update any plugins they sell themselves which allow them to sell the products at a greatly reduced cost than if you purchased from the original developer.
While it is totally legal to obtain a copy of plugins and themes from these stores, it is up to you to decide whether it is ethical: Something to bear in mind though:
When you release a product with a GPL license, you are specifically saying it is ok to sell or giveaway your product for free. That’s what the license means, so it is not unethical because the licensee has specifically told you its fine to do that.
If you are just starting out as a blogger and don’t want to spend a thousand bucks just to test the waters, buying plugins from one of these providers might help save you a lot of money. Most of these providers do not offer any kind of support for automatic updates. That means you will have to manually install plugin updates by first downloading them from these providers and then installing them on your own.
Whether or not it is unethical to sell plugins developed by a 3rd party under your own name is up to you to decide, but by the letter of the law it is not – theme and plugin developer choose to offer their products under the GPL license which specifically gives people permission to resell or giveaway their products if they want to.”
Read more about WordPress’ GPL License: