The law is a funny thing where depending on how contracts and legal documents are written, you can skirt around it by finding loopholes. Blizzard found out the hard way when the Defense of the Ancient custom game built using the Warcraft 3 campaign creator, ended up in the hands of Valve who ended up creating Dota 2.
Blizzard then later had to come up with its own game, Heroes of the Storm, which unfortunately did not quite take off the way the company had hoped. Fast forward to today, Blizzard is learning from their lessons and it seems that in an update to the company’s Acceptable Use Policy, it looks like Blizzard essentially owns all future player-made custom games that were created through its games.
Basically what this means is that while developers are more than welcome to keep creating custom games through the campaign editor, Blizzard more or less “owns” it, which in turn prevents other developers from creating similar standalone games that would be a derivative of it, such as using the same name, art, or characters that would have otherwise belonged to Blizzard.
That being said, Blizzard understands that developers need to make a living, and while they don’t allow developers to engage in direct commercial exploitation of their creations, they will allow them to accept donations that would basically allow them to recoup their time and resources involved in creating these games.