Graduate students and professors need to "publish or perish". I'm hoping that at least some of them will use at least some of their publishing time to write free textbooks.
It is unlikely that a free textbook (or any textbook, really) will count for tenure, in either the social or physical sciences, barring very high level technical textbooks.
So, yes, they would be wasting their time unless already well established. That rules out grad students and early career professors.