This is true. But that does not mean that the publisher provides minimal value; it's worth remembering that Knuth lobbied hard to get the Journal of Algorithms moved from Elsevier to the ACM (where it became TALG) due to issues of copyright and pricing.
The ACM (and IEEE) are miles above the commercial publishers in this regard.
Plagiarism, or detection of (academic) plagiarism has very little to do with copyright.
The ACM says otherwise, and they are the ones who are actually pursuing cases. I tend to take their word for it.