I must admit, I did not watch the video, had just read about it and did not know about the assault.
That is of course a crime. I am not sure that a postal worker would be considered a federal agent since it is an independent, federally backed corporation, so it may or may not be a federal crime as well.
The recording is still illegal.
Looks like you are right and the recording is illegal in Mass. That law to me is pure garbage in this case. The guy is on a public street and should be able to tape anything he wants whether it is secret or not. Here is the law:
http://www.citmedialaw.org/legal-guide/massachusetts-recording-law