I'm curious about their reasoning for choosing Creative Commons Zero over the public domain. I'm fond of the public domain; I've released some of my software[0] into the public domain. Cynically, I think it's more fashionable and branded to license something CC0 than declare it public domain. Despite my cynicism, I like that they're doing this. The article says: > Enabled by the Museum's move to open access, we also…
It is possible that the Met has licensed some|many|all of the images from their authors. If so, then I would not be surprised if the terms of such licenses are grounded in copyright and while the licenses may grant very permissive usage and relicensing rights, the licensing rights are unlikely to have provisions allowing the Met to terminate the author's rights on behalf of the author as would be required to place a work in the public domain.
Creative Commons exists to avoid some complexities that come with placing work in the public domain when copyright would otherwise obtain.
But again IANAL.