I guess things would be far more easier if WikiData would choose a
CCBYSA-compatible database license like ODbL. I can only repeat myself,
that the OpenStreetMap community went through a similar discussion and
decided to respect database rights and keep their data as clean as
possible. Because they didn't want to take the legal risk, that after 5
years a big company like Google will sue them and so OSM might loose the
work of 5 years.
I also think that there are two aspects in this topic: 1) the pure legal
aspects and 2) how we (Wikipedia users) treat each other and respect the
work of others. So the WMF might say: hey you European guys with your DB
rights... bad luck that you invest all your work into a US hosted
project - we don't care about your rights. Even _if_ this is legally not
to be objected, is it fair against the users?
To make it clear: I'm into lots of list articles (streets and monuments)
which contains lots of data. This content is licenced under CCBYSA
today. The day when all this content will be expropriated by WikiData,
is the day I'll stop my work for these Wikipedia projects.