Citus owns the copyrights, and the CLA ensures that they will continue to. That means that they can reassign it under a different license if they so choose, and they could at some point choose a liberal license.
And a CLA in place also means many contributors will shy away from doing so because they have no idea what happens with their contribution in the next version. Either it's a community project owned by everyone under an agreed upon license or it's more like a record deal for new artists.