Purpose of IP address and timestamp on open-access full-text article
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.