Having read the @mokusatsu links, I now think these are probably similar to the Language Residents they have at Pomona. At Pomona, their duties are described as:
http://oldenborg.pomona.edu/opportunities/prospectivelr/. The Pomona Language Residents also have to take 3 academic credits to qualify for their visa. I agree that it sounds far fetched that Amherst would be pressuring its language assistants to sleep with students, particularly given the recent focus on prevention of sexual assaults on campus.
If a grad student has completed all comp/relevant foreign language/technical skills(relevant stats, programming languages, etc) exams, submitted a dissertation topic to adviser/department, and had that topic approved…a doctoral student doesn’t necessarily need to live on or even near campus so long as he/she registers him/herself each semester to maintain grad student status and periodically checks in with the adviser on progress of the dissertation thesis.
For instance, some the Profs I had started their careers while still finishing off their dissertations from institutions hundreds or even thousands of miles away. Also knew a Harvard PhD student in the dissertation stage living in NYC partially because the vast majority of his research/resources pertain to certain NYC neighborhoods.