Most Open Source code comes with a requirement to carry over license note, which Copilot does not do. Additionally, ethics dictate you attribute the source when copying directly, something the Copilot also doesn't do.
It's already been demonstrated that Copilot - like all tools in the GPT family - frequently output large chunks of their training dataset verbatim. It's not hard to trigger this behavior, even unintentionally. To me, this is much closer to "reusing". But I'm not a lawyer.
It's also worth remembering that there are two parties potentially open to liability here - GitHub, with the way the code was used with the Copilot, and the user, who may be unwittingly including licensed code in their codebase. Given the well-known behavior of the GPT family I mentioned above, it might be hard to argue that Copilot "just chanced" into generating code that's identical to existing, non-public-domain code.
Ah, that then is extremely problematic.
I really like the idea of Copilot to speed development-- basically code completion taken to an extreme-- but this seems like a very bad way to go about it.