It is not against the law to alter or redistribute a WordPress plugin or theme that was distributed under the GNU General Public License. However there are many who believe that it violates ethical standards.
Because the GPL license gives you the right to alter and republish the work of other developers under your own name it opens the door to the possibility of various forms of misuse. Particularly GPL clubs like GPL Vault have been singled out as some of the most egregious violators of the GPL s open source spirit in recent years. People don t always have the financial means to pay for dozens of pricey WordPress themes and plugins or they simply don t want to pay for them. This is the ostensible reason why these stores exist. If you are unsure about whether or not a specific product will fulfill your requirements it is reasonable for you to act in this manner. Therefore you should regard the websites such to GPL vault to be try before you purchase retailers.
You can easily test a wide variety of competing plugins by subscribing to the GPL club. This allows you to determine which one is the most suitable for your project. Because GPL Clubs like GPL Vault or are concerned with aggregating software and nulling it (removing its licensing limitations although at Plugin Rush we do NOT sell any nulled plugins) they are able to sell these items for a price that is significantly lower than what it would be if you were to acquire them directly from the original developer.
Again obtaining a duplicate of plugins and themes from these marketplaces is perfectly legal but some people believe that it is unethical to do so because you are robbing the original creators of the opportunity to make a career off of their hard work.