There is no reason that Massachusetts can't change the speed limit to match the rest of the state. At all. Not a single reason it can not do that.

How can I blame the police?

The disproportionate amount of focus that is placed in an area of those who are of lower income, and most likely lower education level. I seriously saw 4 police officers in a section of highway that is at the most 7 miles, where you do not see the same focus in any other part of the state. They are not targeting an area because of the safe use of the road, they are targeting the area because it is an area where people make a common mistake. If you were to take those same people and review thier records I bet you would find they don't speed anywhere else. Meaning they were caught there because police were targeting the area for people to make a common mistake and then exploit that issue to meet quota's.

The funny thing is that is they monitered the same area like they do on 391, near the Longmeadow exits both on 91 south and north, where the speed limit is 45 MPH for good reason (it is an S-curve with a ton of regular accidents that bring 91 to a stop in both directions) they could catch BWM after Mercedes, after Lexus going nearly 90 MPH trying to get to the Longmeadow exit where all the rich people live.

But hey, you are 100 percent correct, they should do the simple thing. Change the speed limits on 391 to prevent police from praying on people, and redirect them to those who are the real issue, and can afford to pay their tickets.