That's Massachusetts law. The city of Boston has its own additional restrictions. Here:
16-45.1 Carrying of Weapons Prohibited. No person, except as provided by law, shall carry on his person, or carry under his control in a vehicle, any knife having any type of blade in excess of two and one-half (2 1/2") inches, (except when actually engaged in hunting or fishing or in going directly to and/or returning directly from such activities, or any employment which requires the use of any type of knife), ice picks, dirks or similar weapons that are likely to penetrate through police officer's ballistic vests, or other object or tool so redesigned, fashioned, prepared or treated that the same way may be used to inflict bodily harm or injury to another.
16-45.2 Penalty. Violators of any provision of this ordinance shall be subject to a fine of not more than 300 hundred ($300.00) dollars for each offense.
16-45.3 Severability. The provisions of this section shall be severable and if any section, part, or portion hereof shall be held invalid for any purpose by any court of competent jurisdiction, the decision of such court shall not affect or impair any remaining section, part or portion thereof.
So I'm wondering how the hell the average citizen is supposed to know what will penetrate a ballistic vest.
This part:
"or other object or tool so redesigned, fashioned, prepared or treated that the same way may be used to inflict bodily harm or injury to another."
Is almost beyond belief...