The direct exporting of resources from games other than Warcraft III is prohibited. This prohibition includes, but is not limited to, World of Warcraft®. No such resources may be submitted to THW. Forum discussions pertaining to the export or editing of resources from other games will be closed.
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The Hive Workshop allows you to use models and art from other games as long as the copyrights of those games are owned by Blizzard Entertainment.
The logic behind this is that, by law, you are not allowed to just take artistic property from other companies and use it in your games. Since Blizzard now deems that any StarCraft II (SC2) user-created maps are their property, using a model from, for example, a game made by Electronic Arts (EA) in a SC2 map is equivalent to Blizzard taking a model from EA and, without permission, using it in SC2. This would probably result in EA suing (taking legal action against) Blizzard.
Since Warcraft III and StarCraft II are both Blizzard games, Blizzard is most unlikely going to sue itself because you exported from one game to the other. The Hive has rejected exports from World of Warcraft until September 28 of 2011 because the legal agreement that you tick as read-and-acknowledged upon installation of any game by Blizzard states that you are not allowed to export from the game at all, but if Blizzard is not enforcing its own copyright rules within their games, then the Hive does not have a reason to.