[Phanx]

Short answer: Yes, it is legal to restrict editing of an addon. In fact, editing is prohibited by default under copyright law!

Long answer: The only thing the WoW EULA says about addons is that "Blizzard may, at its sole and absolute discretion, allow the use of certain third party user interfaces"... but even that is located in a paragraph forbidding the use of "third-party software that intercepts, "mines", or otherwise collects information from or through the Game or the Service" so I'm not sure that "user interfaces" in that context actually even refers to addons.

Copyright law protects addons, which are software, the same way it protects songs, movies, novels, poems, paintings, and all other creative works. Unless the copyright holder of the work (usually its creator, unless the copyright has been transferred to someone else, such as the publisher of a book, or a recording label) specifically gives you permission to modify and redistribute the work, then it is illegal for you to do so.

Please note that all works to which copyright law applies are automatically copyrighted at the time of creation. The author doesn't need to register their work anywhere. They don't even need to include a copyright notice with the work. By default, it is "all rights reserved" unless the author specifically waives some (or all) of their rights under copyright.

The copyright holder may grant permission to everyone through a license, or they may grant permission to an individual entity by communicating in writing that they give such permission.

If the addon doesn't include a license that clearly gives you permission to distribute modified versions of it, then you cannot do so unless you get written permission from its author.

However, copyright does not protect ideas. If you cannot get permission to distribute a modified version of an existing addon, you are free to write your own addon that does the same thing, though you must actually write your own code -- merely copying the code from the existing addon, changing a few variable names, and giving it a new name doesn't count.