I'm not sure why you find it funny that the complaints come from copyleft advocates? The entire point of copyleft is that code is shared only the condition that later modifications are also shared back. Copilot strips the license from the code it generates, so this condition isn't imposed on derivative code, which defeats the entire point of copyleft.

If someone was okay with the shared code being used in a different context and changes not being shared back, they'd have used a permissive license. But they did not.

The reason copyleft people are up in arms about this is because it entirely undoes the social and legal contract of copyleft licenses.