Programming is art, and no copying code is not the norm only bad programmers do that. Using ideas from other people's code is done though especially with math and other complex algorithms which are infeasible to derive yourself, and this is "taking inspiration" from other people's work by building on top of it when you incorporate such ideas into your own code.
Also no, people open source their code to also help their fellow skilled programmers not just less skilled ones. Being able to see the source helps people understand the software better, fix bugs in it, and use those ideas in their own software.
Finally it depends on if you care about copyright or not. Secrecy is not copyright. You have the right to keep code private yes, but if you post it publicly online then it's fair game for anyone to download (if you are like me and do not believe in copyright, because you cannot own information). Art however inherently requires posting to the internet usually for people to see so it's always fair game to copy (though this does not mean you need to publish the PSD file).