Publishers of open coursework, including textbooks, are generally supported by academic institutions and philanthropic foundations, and those who write them do so with the express purpose of making the materials freely available for use by other professors and students. This way, not only is the material available for anyone who needs it, but it's also easier to continually update and improve the information available within.
The Code of Best Practices below also explains how educators can use certain materials under copyright for educational purposes as OER.
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