What a questions...Since Blizzard got in their Useragreement that you arent allowed to edit things that are made by Blizzard (baiscly whole World) it is Illegal.But Blizzard doesnt care about Editing and Modelling as long as you dont get them onto a Server that is big and could be made Profit problems for Blizzard ( xD) . I also heard that you just need to say that you used Blizzard Tiles and Objects and not your own, so that the Copyright of them still exists.So in full words:It is illegal, but Blizztard doesnt care