Is it legal to restrict editing of an addon?
 

It is allowed by the EULA to restrict users of an addon to make modifications to it and re-releasing it?

It may not be part of the EULA, but it would possibly be against copyrights depending on the addon.

It depends entirely on the license under which the author has released their works. Also note that if no license is included, it is automatically 'All Rights Reserved'. Check for a license to see what the author permits.

I noticed an addon explicitly included a licence that prohibits modification. I was wondering if Blizzard allows them to do that.

Blizz has no say in the matter. Addons are the intellectual property of the people that write them.

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.

I don't see why people feel the necessity to release their mods of things. I've never understood it, even when I was involved in the Windows modding community. Then if the author disallows mods, people still float it around vie e-mail and post screenshots and crap to show off. Someone releases a theme, I dislike some aspect of that theme (or the author did half-assed work on some part because she/he never sees it during their own use of Windows), and I either mod it to my taste or put in some "fixes." I did that with dozens of Win XP Visual Styles over the years and released a grand total of two because (1) the author specifically stated that the theme was open source, and (2) the changes were so grand and sweeping as to almost be a completely different theme altogether.



(Ooh new "highlight lua button." Nice forum addition!)

In most legal documentation, there is a difference between pre-compiled binaries ("Software") and "Source" code. These terms exist to describe different stages of a program's development. In this case, "Third-Party Software" refers to any external pre-compiled binary and "User Interfaces" refers to any external "Source" objects (scripts, artwork, sounds, etc.) that are to be used directly by Blizzard's addon system.

As a note, both "Software" and "Source" are covered by copyright law and in the case of "Source", even specific chunks of code can be individually copyrighted.