Yes, it is. Using a library is a transition of abstraction layers. It's expected that some understanding might get lost at that point. It's also expected that you understand what the library promises to do when you make that call - i.e. that you read the docs.
Copy/paste lives at the same abstraction layer as code that you actually write yourself - the same level of understanding should apply. (Of course, if you don't understand any of the code you write, you're in the clear ;)
In that case, it sounds like it just depends on whether you paste the code directly into an existing file. If you paste the code into its own file, then import it into your project, the distinction seems to vanish.
A library isn't just code that happens to be in a separate file. Someone chose an abstraction layer at which to create a library, wrote the library code, tested it and wrote documentation.
If you went and copy-pasted some code on stack overflow into a file, put it on github and released it as a "library", I might not agree that it was one.
Copy/paste lives at the same abstraction layer as code that you actually write yourself - the same level of understanding should apply. (Of course, if you don't understand any of the code you write, you're in the clear ;)