You wouldn't have to. Without copyright, it seems to me that everything becomes copyleft by default.
I could, for example, modify any piece of software I like, and legally distribute modified copies to others. Even if I didn't want to share my modifications with a larger community (or didn't want to do it for free), I couldn't stop it - someone else could do that for me. And as 'wolfgke points out, being open source doesn't matter much here.
I could, for example, modify any piece of software I like, and legally distribute modified copies to others. Even if I didn't want to share my modifications with a larger community (or didn't want to do it for free), I couldn't stop it - someone else could do that for me. And as 'wolfgke points out, being open source doesn't matter much here.