Defeating a copy protection measure is illegal, even if the copy protection measure is not copyrighted.

I'm not talking about the copy protection, but the software being protected.

Surely if the work being protected isn't copyrighted, there's nothing to circumvent in terms of the DMCA?

A non–copyrighted work can still have a copy protection measure

But would the DMCA ("Copyright Act") still apply when there's no copyright?

Here, let's read together: https://www.law.cornell.edu/uscode/text/17/1201

> No person shall circumvent a technological measure that effectively controls access to a work protected under this title

> protected under this title

A work not protected under title 17, copyright, is thus not covered.