The museum's purpose is to preserve the artwork and scanning it furthers that purpose.
Their purpose is not to display the artwork and sharing the scans would take away from their purpose. If other people are already preserving the artwork, they have no need for you to do it too.
Surely their purpose is both? I haven’t read the specific stipulations Rodin put on this museum he founded but I assume he didn’t mean for them to not display his works.