BTW, it's not only copyleft code that is a problem. "Permissive" licenses also have compliance requirements, such as attribution or preserving the original license text. Only code under Public Domain/CC0-like licenses can be reused without care.
Even that isn't true, at least for pure public domain without a fallback like the CC0. Some jurisdictions (like Germany) don't allow authors to place their works into the public domain.