1) Your analogy is wrong, because none of the software licenses forces a user to have a license for his work. Moreover, a math book has strong copyright protect

1) Your analogy is wrong, because none of the software licenses forces a user to have a license for his work. Moreover, a math book has strong copyright protection. You cannot just copy the math book and sell it as your own. GPL allows that. 2) Yep, the whole idea of GPL is to allow those good sources to propagate freely and get reused. Read the license, please. 3) Citation model is the core of BSD license, so use it for your own code.