This conversation's going nowhere because we've started with a one-sided news report and all of us agree that this is bullshit. It's chest-high full of straw men in here. So here's an attempt to describe the SWAT teams' legal rationale for this. It comes from reading the news report, reading the Law Enforcement Councils' website, and living in Massachusetts so I know how local government works. Plus a lot of "what would I do if I were evil" speculation.
Massachusetts has very weak county government. Local law enforcement, even in rural areas, happens at the town level. Many small towns have like one cop car and two cops, and can't afford a crime lab, a drug lab, a K-9 unit, and whatnot. The county provides specialist services to all the towns within it, but SWAT teams are not one of these services. It makes total sense that local towns would form a cooperative association (the LECs) to pool their limited SWAT resources.
From what I read on their websites, it looks like SWAT personnel don't work for the LEC. They're ordinary town cops who're assigned duty to work with other town cops through the LEC. That is to say, the LEC has no law enforcement authority, but the individual cops do. The LEC provides equipment storage, networking, and shared training for the town cops. As far as I can tell, legally, the LEC is just a place to park the SWAT van, a seminar room for Powerpoints, and a phone tree.
It's not a private army, I bet they'll say, it's a professional association, just like how your city's dentists all get together at the country club first Tuesday of the month. All the personnel are employed by the towns. All the equipment belongs to the towns. You want the public records for your recent 3 a.m. visit by the battering ram boys? Talk to your town.
Now, clearly, this is all bullshit. But it seems to me to be well-crafted bullshit, created by the SWAT teams' lawyer buddies, and we're not going to make it go away unless we appreciate it for what it is.