Material that is published under any license at all is copyrighted, and that copyright ought to be formally registered with the government. Whether or not it has been registered, however, it is "owned." And, "ownership has its privileges."
One of those privileges is that the owner can choose to license it any way he likes, and he can change that license at any time. When someone "forks" the code and does something else with it, they are creating what is called a derivative work. The creator of a derivative work is entitled to obtain his own copyright on the derivative he has created, but it is subordinate to the original work. There are certain things that he can do, and others that he can't. For example, he can't do what this person attempted to do: namely, to replace the copyright terms with a less-restrictive (or even, "meaningfully different") one, thereby compromising the legal rights of the owner of the intellectual property from which the derivative work was derived. The owner had a legal right to demand the change, and the offender – who was in the wrong – had no choice but to promptly comply.
Although these copyright licenses are designed to be "open," they have sharp legal teeth. They have been tested and upheld in courts all around the world. When you own something, you can be as "open" or as restrictive as you care to be. You still have enforceable rights.
See this article by an IP attorney for a much better discussion.