You're correct that copyright does not apply to ideas only implementations. However if you take an existing implementation and base yours on it, generally the original work's copyright applies (there are a whole lot of details this is skimming over).

As an example, if you take a painting someone else made, and try and make your own version using the original as a reference, that is probably subject to the original author's copyright. On the other hand if you both happen to paint the same sunset its all ok.

I think you're more stating how you would like copyright to work, not how it actually does.