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.

Libertarianism on the other hand is primarily about the rejection of personal restrictions

Libertarianism is closely aligned with strong property rights, and the right to exclude others from your property, the right to enter contracts, even oppressive contracts, that you've agreed to. In other words, the freedom to bind oneself and exclude others. Libertarians don't believe in granting the public freedom to walk onto private land and pick fruit from its trees (or the right to harvest lumber on someone else's land).

Strong copyright is therefore fairly associated with libertarian ideals.