Wikidata is becoming a repository where people may choose to share their data .. or not. When they do not want to share their data with Wikidata, it is their choice and that is fine.
The bottom line of copyrights for databases is that single facts cannot be copyrighted. It is only the whole of a database that can be under a copyright. When you look at the data of Wikidata and its structure, it is in many ways a reflection of all the Wikipedias. Increasingly its data finds its way into Wikidata and as a consequence data that may be found in a specialised database gets included in Wikidata.
Wikidata also has the habit of including identifiers to external sources on an item level. As a consequence people can see what other sources have to say about the same source. It also enables bots to make a comparison. When it writes a report about the differences, it is original research and consequently it does not violate any copyright. When based on such a report people make changes, it takes an effort to find what is correct and consequently it does not violate copyright either. When an agreement is in place, it is possible to add missing data to Wikidata. When done properly there will be an attribution of the original source and, when it is done by a bot, it may be a bot dedicated to that resource.
The objective of the Wikimedia Foundation is to share data. This is why it makes so much sense for Wikidata to have a CC-0 license. As the quality improves, as more and more comparisons are made and the differences are reconciled the data becomes more valuable. Given its scope, not much is out of scope and it is obvious that Wikidata needs to include data from other sources wholesale. It may get information in so many ways. With the CC-0 license it is obvious. Use our data, compare our data, improve our data and this will bring more power to us all.