By the "ornery cop" logic, what would stop him from just locking a speed into the RADAR or LIDAR gun at the beginning of his shift, and then showing it to everyone he stopped all day as the speed they were supposedly clocked at?
Why not take it one step further and say that unless they have a video tape of someone committing a violation, they can't take enforcement action?
In reality, for many reasons including the one above, it would prove nothing for the violator to see a number displayed on a readout.
In addition, in many cases, the officer doesn't lock a target's speed reading into the machine at all. It is much more important for the officer to maintain what is called a "tracking history", which means he monitored the target display during the entire track and can verify that the indicated target speed changed up or down as the audio tone changed (the Doppler signal pitch and intensity), and that observation correlated with his observations of the target itself (visual speed estimation). Those are all key ingredients in target identification, or, in other words, making sure you're tracking the right target.
So no, they don't have to show you a locked-in speed indication.