Keep in mind these are "early versions" of documents that will be cleaned up for publication: "Early Access offers readers a backstage pass to a phase of the documentary editing process never before available." I.e. they grant full access to early versions, and remove it from the Early Access site (and move it into the "Rotunda" service,
http://rotunda.upress.virginia.edu/) when it's ready for publication.
I'm not clear on copyright as it relates to this situation, since we're dealing with unpublished works held by a private collection that are apparently revised, proofread and edited. Even if they can't copyright it, they certainly can restrict and charge for access to Rotunda, though if the works are not actually copyrightable they couldn't stop you from subsequent uses and distribution.