But I'm going to play devil's advocate, because I want to know the truth:
I wonder if there are other, possibly more important reasons for forming these private corporations. Maybe the existing police system doesn't function optimally, and provisioning resources (such as trained officers with appropriate equipment for drug busts) is a process that's too slow, or inadequate. Maybe this is a way of detouring the bureaucracy and systemic bullshit that encumbers civil servants whose job is ultimately to keep people safe.
The article says that Tewksbury, MA paid $4600 for membership to NEMLEC. That town has a population of around 28 000. It doesn't make sense for a small town like that to have a police force with a dedicated SWAT team, computer crimes unit and schools incident response team. It also doesn't make sense for a larger jurisdiction to serve the smaller community when it [the smaller community] needs it, and get nothing in return. It seems as though it's a way for police departments to share resources. I'd imagine that if a small town's police department found themselves unable to deal with a time-critical scenario, like a shooter in a school, they'd call in backup pretty damned quickly. Perhaps the quality of response a small town could get through NEMLEC would be better than going through traditional police channels?
To me it seems like the primary purpose of forming these private NPCs is not secrecy. As the article says, government police agencies already do that - "police agencies have broadly interpreted open records laws to allow them to turn down just about every request."
So, if it is actually easy for police agencies to turn down requests in the first place, why go to all the effort to form, finance, and manage a 3200 member [1] corporation?
[1] http://www.nemlec.com/who.htm
edit: clarified a sentence by adding 'through NEMLEC'