That's a little simplistic. From a legal perspective copyright is a state enforced monopoly. Perfectly reasonable for a libertarian to reject it.

In the same way, someone on the left could view it similar to the English enclosures. Taking for a private something that should be in common.

Ideologically speaking it can be perfectly plausible to reject the current overdone copyright regime from either perspective.

And by the way Doctorow talks about collective bargaining and unionizing like the whole time.

I can also recommend you a great book by a Chicago economist that argues against copyright on lassez faire grounds, closer to the libertarian position, but really well supported with a trove of historical case studies.