"Tainted rewrite" isn't a legal concept either. You have to prove (on balance of probabilities - more likely than not) that the defendant made an unauthorized copy, made an unauthorized derivative work, etc. Clean-room rewriting is a defense strategy, because if the programmer never saw the original work, they couldn't possibly have made a derivative. But even without that, you still have to prove they did. It's not an offence to just not be able to prove you didn't break the law.