The Cunctator, 18/06/2018 17:57:
Is there a coherent explanation of what content Wikidata is extracting from CC-BY-SA projects?
Not really, because it's a very distributed and long-running process (which is very very far from completion). But there are some relevant examples explained in some publications. https://eprints.soton.ac.uk/412922/1/opensym_wd_vs_wp_2_.pdf https://eprints.soton.ac.uk/413433/1/Open_Sym_Short_Paper_Wikidata_Multiling... http://oadoi.org/10.1145/2872427.2874809
Your claims that the abusive destruction of copyleft by the Wikidata project have nothing to do with copyright and nothing to do with Wikidata don't make any sense.
By "abusive destruction of copyleft" do you mean the practice of mirroring Wikipedia articles, or snippets thereof, without really complying with the copyleft licenses?
The first large scale example (which went way beyond the usual rogue mirrors) was probably Facebook with "community pages" in 2010. https://lists.wikimedia.org/pipermail/wikimedia-l/2010-April/104232.html
At the time, the Wikimedia Foundation attempted to get them include a visible reference to CC-BY-SA, links from which the history and edit buttons would be visible, and so on. Some considered the result acceptable, some didn't. But many said there was no way to enforce something else.
Free riders are a common and well-studied issue of copyleft projects. The countermeasure is generally some kind of copyleft compliance syndicate, like https://sfconservancy.org/copyleft-compliance/principles.html
Federico