Do you realize that in some jurisdictions, you can write out a complaint and summons for that parking violation? If it is a civil offense, you don't need to be an agent of the state to commence the action.

Take photographs of the car, the sign, and submit the paperwork at the local municipal court. Give the summons to another adult to serve upon the state at police headquarters. You might need to do some research the first time around, but after that, you can ticket that car again every time you walk past it instead of silently fuming at the double standard.

Cops have to obey the law, too. If they won't obey it and enforce it against each other, that leaves it to the rest of us.