Because they law makes a criminal of someone with no criminal intent.
I am 61 years old. I have carried a penknife with me since I was 10. I now find that I am a criminal to do so because it could be used as a weapon. By the same logic, we should have a law that makes driving a car a criminal offence. When I ride my bicycle or drive my car, I carry an assortment of tools for use in the event of a breakdown. One of these tools is a multitool that has a blade. I find myself a crimimal once again. This is a nonsense. It would not be so bad if the police used common sense, but they are put in a position today of being more concerned with the letter of the law rather than the spirit.