Legal fine print that gives a publisher a non-exclusive license to distribute mods is common. If you create a mod with the Skyrim Creation Kit and distribute it, for example, you "grant to Bethesda Softworks the irrevocable, perpetual, royalty free, sublicensable right and license under all applicable copyrights and intellectual property rights laws to use, reproduce, modify, adapt, perform, display, distribute and otherwise exploit and/or dispose of" your mod however it sees fit. (Lawyers, eh?) In short, you've given Bethesda license to do whatever it wants with your work. If Bethesda wanted to, it could package up your Skyrim mod and sell it as an official add-on without compensating you. (Not that Bethesda would likely do that, even if it reserves the right to.)
RE: Blizzard's claim that it owns Warcraft 3 mods isn't just the standard fine print