In the US, they can't. See Bridgeman vs. Corel. Despite whining from the museum community, nobody has successfully enforced a copyright on a public domain work in the US since then. This follows the Supreme Court decision in Feist vs. Rural Telephone, which wiped out copyright in telephone books as not being creative works. Bridgeman was followed by Meshwerks vs Toyota, which ruled that 3D scans of an object are not new works.