Purpose of IP address and timestamp on open-access full-text article I understand that some pay-walled journals include timestamp and IP address and/or institution to full-text PDFs to track copyright violations.

However, I also saw an open access journal doing the same despite allowing non-commercial, attributable redistribution and modification.

If that journal grants such rights to readers, then why does it include IP addresses on the copies they download?
Most likely, they are still doing it to track copyright violations. If the document is made available under a license that permits non-commercial, attributed redistribution, then the publisher might wish to collect evidence in cases of commercial or unattributed  distribution.

It's also possible that the same publisher, or even the same journal, also has articles that follow the traditional "all rights reserved" model, and that this information is applied to all downloads indiscriminately.