Sure, sure, Boston English has different pronunciations and vocabulary than English as it is spoken elsewhere, but it also has some unique grammar as well.
A few years back (but just now reaching us here in the UHub cave high up on a ridge on the Roslindale/Hyde Park frontier), the Yale Grammatical Diversity Project took a detailed, scholarly look at our unique negative positives, such as "So don't I!" Or what linguists apparently call "implicature canceling:"
[W]hen speaker A explicitly states that he/she plays basketball, he/she also implies that speaker B does not play basketball. Then, by saying, "So don't I," speaker B asserts that he/she actually does play basketball, contrary to A's implicature. Thus, although So don't I is affirmative, it also has a negative function in that it negates an unspoken assertion.
That seems to be a rather dour view of the Boston psyche, but maybe those scholars had just returned to their office after having gotten cut off on I-84 by a Masshole.
Apparently, the northern and southern boundaries of the usage go from York, ME to New Haven; a western boundary isn't set, but presumably it's somewhere east of Stockbridge. Still, like "bubbla" in Wisconsin, there are some exceptions, but there's a good explanation:
Lawler (1974) reports the phenomenon in DeKalb County, Illinois. In a posting on Linguist List, Lawler states that many "many early settlers of DeKalb County originated" in New England.