Authors are often allowed to share at least their preprints (i.e. submitted manuscripts that have not been peer reviewed yet) elsewhere, but this rarely happens in many academic disciplines. In fact, they are often required to do so by their institution, to deposit their work in its Institutional Repository, yet even that requirement is often flaunted.

(In my view, this is because the personal benefit is not that clear yet, especially if the work is just hidden in some long list of other research. Which is why I'm working on helping them use their research to raise their academic profile: https://medium.com/flockademic/the-holy-grail-in-open-access... )