Sigh... I did not say anyone could actually opt out.
I said what someone who did not accept the very premis of copyright would or should logically be expected to do.
They would not participate in the thing they don't recognize. If everyone else gives their works a default copyright, that's everyone else's problem, they didn't do it.
But GPL is ver definitely not that.
It is not subverting or perverting or sabotaging copyright, it IS a bog standard employment of the right to set the terms over a thing that you believe you have the right to set the terms for, and you have very definite ideas about what you want and don't want other people to be allowed to do.
When I choose GPL for something, make no mistake that the reason I do so is because those are the terms I want to enforce. I want to impose exactly the limits it imposes. That is my payment for the product. I am using copyright for it's intended (or at least advertised) purpose, not trying take a notch out of the concept of copyright because I don't believe in the validity of copyright.