To release some content under a given license, one must in legal position to release the corresponding material under the chosen license.
So far, as far as it is know to the author of this ticket, Wikidata didn't apply a strong process to enforce conformity of imported material with its policy of CC-0 as single license for data stored in the relational model of its Wikibase instance.
This put Wikidata at best under strong legal uncertainty, as some contributors, as already exposed, do perform massive import from various data banks. Indeed, regardless of the "factuality" or "creativity" of each single data which are part of this massive transfers, this raises concerns due to existing monopoly granted to data bank creators, whether through specific rights on databases (ie. in Europe) or through the extent of copyrightable material (ie. USA).
This makes Wikidata current state not only a concern for the whole Wikimedia ecosystem, but also for any downstream stakeholder which would be interested to use Wikidata, but not to take the risk of currently attached possible legal threats.