How can you say people might be pirating it, if you say that wpplugins.com requires that plugins are released under GPL? In fact, I am wondering how this works

How can you say people might be pirating it, if you say that wpplugins.com requires that plugins are released under GPL? In fact, I am wondering how this works. Are plugin authors providing support as an value added service? Otherwise, what is to stop someone crazy enough to buy all the relevant plugins and release them on their own site(pay or free) ?