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 with no expectation that I'll be credited in derivative works. Lately, I'm leaning to CC0 to make it easier for everyone to make use of my work, no attribution required, even. !copyright