There it is. If you have to ask that question, it follows that co-pilot isn't the right tool for developers to use in the first place.
Would you - or your employer - want to risk a lawsuit because you inadvertently copied code that's actually released under a license with particular terms of use?
"Oh, my due diligence went as far as trusting Microsoft and their product for making sure any and all rights where cleared before their tool provided me code." is a meager excuse in a court case when the original author of said code decides to sue you.
> Little bits of code should not be copyrightable in the first place. I can't copyright a single sentence fragment.
True. You can't copyright single fact statements. But clearly, there are cases where co-pilot suggests way more then that.
It's also true that most people don't really consider how stringent copyright laws really are until they are affected by the consequences of their actions (fines, suits, cease & desist,...).
> it's very rare to use a big chunk of code from copilot without changes
That's a tentative claim.