The crux of the issue: Is the code that is being generated being used in a way that it's license allows? That's it. I'm confident that this problem would go away if copilot said:

//below output code is MIT licensed (source: github/repo/blah)

And yes, the "users" are responsible, but it's possible that copilot could be implicated in a case depending on how it's access is licensed.

Stable diffusion has this same problem btw, but in visual arts "fair use" is even murkier.

For code, if you could use the code and respect the license, why wouldn't you? Copilot takes away that opportunity and replaces it with "trust us".