Comment by saurik

And if you then write a program that is remarkably similar to the one you read, that's copyright infringement. As another reply noted--but without anywhere near enough verbosity--this is not without risk, and people who intend to work on similar systems often try to use a strategy where they burn one engineer by having them read the original code, have them document it carefully with a lawyer to remove all expressive aspects, and then have a separate engineer develop it from the clean documents.