They are just some Braves fans who happen to live in Massachusetts. The Atlanta Braves were once upon a time a Boston team.
A friend of mine’s father—the father is prob in his 80s by now—is a Braves fan, in Salem, MA. Years ago I visited my friend and his dad had the Braves on TBS. “Oh, I was a Braves fan when they were here and stuck with them through the
moves.”
(Similarly there are a few New York football Giants fans in parts of New England—going back to the days when either the N.E. Patriots weren’t around, or the Pats got blacked out on TV but the Giants were not.)