Because it criminalizes normally law-abiding citizens and infringes their freedoms and liberties.
Ordinary, law abiding people have become petty criminals, as defined by the Police, simply by having in their possession a Pen-Knife. I refer to the recent case of a 61year old man being arrested and charged for keeping a Swiss Army pen-knife in the glove compartment of his car. Apparently for occasional use with his family pic-nic.
I have carried a pen-knife for 50 years, for various, non-criminal activities. Am I and others like me criminals? Apparently so!
Stop criminalizing the general public with these petty, stupid and unnecessary laws.