A CC-BY license can be a burden, if you really want to fulfill all the terms of the license, namely: "You must attribute the work in the manner specified by the author or licensor." If there are 20,00 authors, are you really going to find out how each one wants you to give them attribution? It's impractical, so what you end up doing is giving what you think is reasonable attribution. But you never really know for sure.
Even worse, some of the material in Freebase is under other licenses, such as CC-BY-SA or GFDL.
Wikidata data is dedicated to the public domain, using http://creativecommons.org/publicdomain/zero/1.0/
Most Freebase data licensed under a CC-BY license. Details are here: http://www.freebase.com/policies/attribution
A CC-BY license can be a burden, if you really want to fulfill all the terms of the license, namely: "You must attribute the work in the manner specified by the author or licensor." If there are 20,00 authors, are you really going to find out how each one wants you to give them attribution? It's impractical, so what you end up doing is giving what you think is reasonable attribution. But you never really know for sure.
Even worse, some of the material in Freebase is under other licenses, such as CC-BY-SA or GFDL.