Comment by necovek

Depends on how you define "learn": usually, a company wanting to rebuild and publish something under a different license prohibits their developers from having ever looked at original code, to avoid the risk of copying over exact snippets out of their memory accidentally.

Copyright protects only arbitrarily non-trivial parts of the original being reproduced, but that means that you have to be careful with learning from copyrighted material. Programming books will have direct clauses allowing snippet reuse, but not for teaching purposes.