Aren't you innocent until PROVEN guilty?
Disclaimer: I have a prosecutor in my family, so this is based on memory of what I've been told, and not on personal knowledge. If there's a lawyer reading this and it makes you cringe, feel free to correct me.
Depending on the ticket, it may just be an infraction, which is considered a civil case, not a criminal case. In civil cases, the rules and assumptions are not the same as criminal cases. To find you guilty you requires only a preponderance of the evidence, a.k.a. "sure looks like you did it," not proof beyond a reasonable doubt. Also, in some places, like WA, you don't even have the right to face your accuser--the cop's notes can stand in for him and you won't get off if he doesn't show up. It's really a different world.
So, if a cop says you committed an infraction, and if *you* cannot prove that his evidence or testimony is suspect, then the court considers you guilty. Thus, you're basically guilty until proven innocent.
It's a neat trick, considering that tickets are used primarily as a source of revenue for the government that gives them out, and only secondarily as a source of public safety.
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Edit for OP: But, yeah, if the gun really is overdue for calibration, and if you weren't too far over the limit, you might as well fight it. That seems to work, more often than not, given the rapid-fire get-this-crap-over-with nature of traffic court, where any decent reason to make the decision one way or the other seems to be enough to get it made.