The cops need to get the arrest numbers up, so they arrest people who haven’t commited a crime. They’re usually normal people without a lot of money, and they can’t fight the charges, and there are no ‘civil rights’ lawyers waiting to help them out.
“There's no way to rule innocent men. The only power any government has is the power to crack down on criminals. Well, when there aren't enough criminals, one makes them. One declares so many things to be a crime that it becomes impossible for men to live without breaking laws.” - Ayn Rand