As the devil's advocate I'd like to argue that legal is probably more comfortable with Unity's own use of LGPL libraries, because they can ensure that Unity only links dynamically to those libraries. And given how critical these dependencies are to the engine, legal is willing to take the "risk" of allowing LGPL dependencies.

The ban of LGPL in assets on the asset store is probably due to legal being concerned that someone would publish an asset that statically links to an LGPL library and that it would allow anyone to demand the source/object code of any Unity game that uses that asset.

Legal probably sees it as too much effort to vet every asset to see if it links correctly to LGPL libraries and simply instituted a blanket ban to simplify enforcement.