Many graduate departments have some kind of rule in place to prevent graduate students from taking on outside work. The reasoning behind it might seem sound enough at first glance — graduate students are supposed to be learning their craft, which means working on a dissertation and teaching. Ideally, they should be free from distractions while they’re doing this.
I don’t know anybody who feels that graduate school is too easy, and it’s understandable why faculty members might want to make sure their students have the space they need to thrive. Moreover, many faculty members, especially those in the humanities, believe that taking work outside academe is antithetical to the purpose of graduate school, which is to get people ready for tenure-track positions and then into