This is not about the GPL'd code people take from you, it's about what they add/change to this code. Whether you can directly slap the GPL back on these modifi

This is not about the GPL'd code people take from you, it's about what they add/change to this code. Whether you can directly slap the GPL back on these modifications is very discutable. You can most certainly sue them for breaching the license, though. (and, like sister comment says, this is not what's happening here anyway)