I can tell you why...MA has some of the strictest gun control laws in the country.
If the police went out waving around an M4 assault rifle that was already illegal in front of a mostly disarmed citizenry how do you think that would look? Proof that criminals don’t care what is legal and what isn’t. The m4 assault rifle is already illegal for any civilians to own.
Maybe it’s the one the feds lost that I read about a while back.