It may have something to do with MGL CH 40 SECTION 4J https://malegislature.gov/laws/generallaws/parti/titlevii/chapter40/section4j. It describes the joining of governmental units that are not a city or town. I guess you could consider a state university's police department a governmental unit. Especially when they are already fulltime MPTC academy trained and have full police authority/jurisdiction throughout a municipality. It is probably how the sheriffs are able to join the different councils.