Just a point of clarification for some people. You (the addon author) are still free to license your addon how ever you want ie. all rights reserved, BSD, freeware etc. Blizzard is in no way making you make your addon open source all they are is making you do is show your source code (think of a book you read, you are reading the "source" code but you can not copy the "source" code because the licensing say you can't without permission or referencing the work etc).