Andreas Kolbe's thought-provoking piece "Whither Wikidata?" sheds light on several troubling trends regarding the usage of Wikidata by third parties. Google and Microsoft, who secured well over half of Wikidata's initial funding, are now enjoying the fruits of our community's hard work with absolutely no strings attached. No considerations of public good.
As Kolbe shows, Wikidata usage by these companies lacks attribution, and this means end users don't know the provenance of the data they are served, and the community loses potential new editors. We are also harmed in a third way: any modifications made by others to this rich dataset do not return to the community at large. In other words, Wikidata is "free" as in "free labor", as far as Google and Bing are concerned.
Copyleft is the only assurance we editors have that our work will not be proprietarized (privatized, in plain English) down the line by third parties, who only truly care about free culture insofar as they can cash in on it, completely ignoring the spirit of sharing that is the cornerstone of our community.
A solution for this problem would be to move to a copyleft license. The Open Database License (ODbL), for instance, was designed for datasets such as Wikidata, and has been used most notably on OpenStreetMap. ODbL's "ShareAlike" provisions (much like CC BY-SA) would be a tremendous step forward for our project, as it would ensure that Wikidata and its contributors are credited and that any derivations of this work will be released freely for all.
We should not fear vain threats made by those who wish to use us as mere free labor for their enterprises. Wikidata's mission is not "to be the most used dataset in the industry". Its purpose goes way beyond that. We are translating knowledge into structured knowledge.
We should not bend to the power of industry monopolists. No amount of venture capital or ill-disguised "donations" — which are really investments made with certain expectations in return — should interfere with our goal of making knowledge accessible. In this context, accessibility means "trickling down" freedoms; every downstream user needs to have the same guarantees we are granting upstream.
Ideally, this should not be a controversial point. All other Wikimedia projects are copylefted, with the conspicuous absence of Wikidata. Perhaps we should start asking why that is, whose interests are benefited from weak licensing choices and start organizing ourselves to fix this.