Requiring the same license for derivative works is the "-SA" part of CC. If a work is strictly licensed "CC-BY" then there is no need to "Share Alike", and so a derivative work can be released under a more (or less) restrictive license. Hard-core !FreeCulture weenies insist on -SA so that the freedom of people using derivative works is preserved. But for me, I've been using CC-BY w…