The museum in Florence which houses the David, like many museums, has in the past permitted commercial photography and sale of these photographs under licensing agreements. Presumably, 3D scanning and the resulting models could be treated as analogous (since it results in a new representation), but such precedents have yet to be set.
The Digital Michelangelo Project was carried out under a noncommercial, academic research licensing agreement whereby the project team did not pay any royalties for access to the statue and did not receive the right to redistribute the models except for research purposes where access is carefully controlled; in particular, under the current agreement the models themselves cannot be placed on publicly accessible Internet sites. Individual images of the models can be left accessible to the public, but must be accompanied by the following message (in English and Italian):
Notice: The images of Michelangelo's statues that appear on this web page are the property of the Digital Michelangelo Project and the Soprintendenza ai beni artistici e storici per le province di Firenze, Pistoia, e Prato. They may not be copied, downloaded and stored, forwarded, or reproduced in any form, including electronic forms such as email or the web, by any persons, regardless of purpose, without express written permission from the project director Marc Levoy. Any commerical use also requires written permission from the Soprintendenza.
Notizia: Questi modelli elaborati al computer, immagini computerizzate, e fotografiche sono proprieta` del Progetto Digitale Michelangelo e la Soprintendenza Per I Beni Artistici e Storici per le Province di Firenze, Pistoia e Prato. Non possono essere copiati, scaricati da internet su un file, inviati, o riprodotti in nessuna forma, incluso la posta elettronica o il web, da nessuna persona per nessun motivo, senza un permesso scritto da Marc Levoy, il direttore del progetto. Eventuali usi commerciali esigono anche il permesso scritto dalla Soprintendenza.
Since distribution of the models themselves to end users is prohibited, the question of fair use does not really arise. However, the fact that renderings of the models may be published while the models themselves may not can be seen as an extension of the right to "quote" from copyrighted works that is embodied in fair use.