Because anyone, anywhere, anytime, may be knifed by a thug looking to complete a gang membership assignment.  Like in this Queens market.

Because this is the Idiocracy.

Because all manner of demented people are walking around out there.  Not really all that many of them, but even a small percentage is lots of bad people.

Because, if you don’t stop them, they’ll get someone else.

If they don’t get you first.