This is really only an issue for project where you're trying re-implement a whole piece of software, and not when you pick up some "good ideas" looking at a pie

This is really only an issue for project where you're trying re-implement a whole piece of software, and not when you pick up some "good ideas" looking at a piece of code. If you're creating a functional clone of some software and suspect that the owners of that software might take legal action against you, then you'll need to make extra sure that no part of your reimplementation resembles the original source code. Hence the whole "clean room" approach. But if you're just looking at some code and get some ideas and use them in a totally different kind of program, you're definitely not violating the spirit of copyright, and as far as I know no court has ever decided that you're violating the letter of copyright in such a case either.