Then what do they mean "The Witcher 2's assets are allowed to be used freely"?
The Witcher 2's assets are allowed to be used freely.
Skyrim's assets are not. If you want to add something to Skyrim from another copyrighted work, you have to ask Bethesda's permission.
Yes, there are a lot of mods that are, technically, illegal.
@Oof, no its not.
CD Project Red has given free license to use their assets in any way as long as its not for profit.
See above. Mods have locked threads that discuss porting Witcher 2 assets to Skyrim.