Why would the GPL get involved? Copyright is flexible because by default you don't have any rights to use someone else's creative works. They can attach whateve

Why would the GPL get involved? Copyright is flexible because by default you don't have any rights to use someone else's creative works. They can attach whatever requirements they like to it. Here, they've said you can use it for free if you release your own software as open source. Doesn't seem like a problem.