This is obviously a very simple business trick that relies on the naivity of the purchaser. It is obvious that, if the component is GPL, you can get it from any other user (bittorrent, file sharing), redistribute it and even resell it (but keep it GPL).
It is also very obvious that, if there's no other source to find/get it from, if you purchase, there is absolutely no need to pay multiple times for it. You just need to buy it once, and re-use it on all of the websites that you wish.
Even if it has some source of protection, the source code is open, so you can easily remove that protection.
Just out of curiousity, can you please give the name/website of the Joomla! component?
