Regardless as to whether or not the driver would be breaking the law by going 30 in a 25 MPH zone (and the law is not as clear as you might think*), the point is whether or not fear of police reprisals could intimidate free speech. If there are 100 people speeding, and the police ticket only one of them, it doesn’t matter much whether 99% of 100% of the people get away with breaking the law; it matters far more whether than 1% opens himself up to selective prosecution. Also consider that stopping someone for a speeding infraction can serve as justifications for all sorts of other “discovery.”
* In some states, you can be ticketed for driving below the speed of the flow of the traffic, even if you are going at the speed limit. In other states, you can be ticketed for driving in the left lane if you are not passing a car in the right lane and the fact that you are driving no faster than the speed limit is grounds for saying you had no intention of passing of the car in the left lane is not going substantially below the speed limit.
Understood. But making that case could be difficult, especially if the police ticket a few more people that day for the same offense, just for CYA.