If the defendant is found not guilty why isn't the police officer then convicted of perjury?
Because perjury, like the majority of crimes, requires that the act be
willful. A person cannot be prosecuted for perjury based on an honest mistake. The whole "I certify..." phrasing is simply a term of art meaning that the officer is reporting in good faith that is necessary for the ticket to be used as evidence under procedural rules.
Also, a lack of proof is not the same thing as falsity, especially in court where various evidentiary or procedural rules may prohibit the state from presenting evidence in court even if conclusive evidence exists.
Finally, most people who contest tickets don't actually fight the ticket in court. That pretty much never happens. What does happen is that their lawyer calls up the prosecutor's office (or files a motion for discovery) and the prosecutor decides to settle without ever going to court because they don't want to be bothered with having to go to court over traffic tickets. In such cases, there's never even an
opportunity for the officer to "prove" anything because the original charge is never presented in court. The person who received the ticket simply pleads out to some non-moving violation and pays a fine that doesn't involve points on his or her license.
ZV