Archive.org is a repository that has millions of papers, books and archives that are in the public domain, and are freely available. No registration whatsoever is needed.
Conversely, Academia.edu, is a for-profit company aiming to become a platform for academics. They have collected many works in the public domain, but to access any of them, it is necessary to register, or to "connect" using social media (by which you give consent to share your contacts). This is, in my opinion, nothing more than a facebook-type business model. In fact, their advertisment website says it plainly:
Advertise on the largest platform for Academics
Target 62 million registered users and over 25 million monthly impressions
My question is: is it legal to add a barrier ("free" in money terms, but not in information terms, which has value) to information already in the public domain?