This is Stack Exchange Policy, and this is how they are built.
Using the CC-BY-SA license ensures that people will be attributed, and that whatever they post will stay "protected". This is the way Stack Exchange works, and it allows users to contribute without having to double think about how their work will be used. I want everyone to be able to use my work, and I don't want someone taking it and then saying that no one else can use it. I don't want one people taking it, without having to say that I was really the person that solved their theory of the universe: i.e. They don't legally have to attribute me.
If certain users want to do this, I'll just rewire you to a post at Open Source, which speaks of dual-licensing any user-contributed content: Can I Override a Licensing Policy?. As the answer there shows, individual users can dual-license content, effectively allowing the option to use a more permissive license.
This post has been migrated from the Open Science private beta at StackExchange (A51.SE)