Been renting in MA for over a decade. I've never heard of this and as you found out it's flat out illegal in Massachusetts. I'm not even sure how they could legally determine your unit is the "source" of an insect infestation. That's like figuring out who gave you a cold. It's impossible.

Have you asked the leasing office about this? It may just be the case they're using a boilerplate lease across all their properties and didn't update that section to reflect Massachusetts specific law. If that's the case, they should remove it when you ask. If they refuse to remove it, RUN. It might be illegal, but they'll still take your deposit, and you'll have to claw it back in court.