You can skirt copyright in open-source by simply rewriting everything. Since only the expression, and not the ideas, can be copyrighted, rewriting to obtain the same functionality is feasible. Indeed, Google rewrote Java when it developed Android, and even though it retained Sun’s APIs in its own brew, in Oracle v. Google a court found that you can’t protect APIs, as they are basically functional in nature and required for interoperability.
Indeed, the main protection from closed source software is in that the source code is secret—and not that it’s copyright—because the secrecy of the code makes it difficult to replicate the ideas and functionality behind the code. It’s a lot harder to reverse engineer software than it is to massage code.
