What you say above would certainly be true if all the contributors to the Bukkit/CraftBukkit source code had signed over their copyrights (or whatever is the appropriate terminology for the system that Apache and many other open source software projects use). But that isn't the case here. So while Mojang certainly acquired the rights to something, they don't seem to have gone to each contributor and gotten the rights to the code those contributors submitted. My understanding is that the Bukkit team would need to get permission from all contributors before they could change the license. Or remove the code of those contributors who don't agree with the change. What do you think?