It does happen. Not going to bother with linking but....
In Maryland some lady was hit with 2 miles under speed limit, for hogging the left lane. Her excuse that there were 40mph winds that day making for hazardous conditions was denied.
In North (South?) Carolina a stop was ruled valid because 'The speed limit was 60, everyone was doing 75, so the trooper was right in pulling you over for doing 55'. This was an attempt at an appeal on the basis of unlawful stopping pursuant to a possession charge.
In Ontario a man was dinged for doing 17 kmph over the speed limit: 'you are breaking the law if you do even one kilometer over the speed limit' said the judge. He protested a month later by doing the exact speed limit, causing a 4km long traffic jam. He was hit with obstructing traffic and had his license suspended. Of note: he had a 'partner' that was nailed with the same fine, who was driving in tandem in the adjacent lane.
Ontario has a section of it's Highway Traffic Act (132) that states basically 'don't slow or block traffic unless there is a damn good reason' - 110$ fine and 2 demerits.
Lastly, someone in Australia was nailed by photo radar for 'allegedly doing the speed limit in a speed limit zone'. That one I'm pretty sure was dismissed as a stupid glitch.
Basically people do get nailed for driving too slow, but it's generally because they are interfering with traffic, or in a few cases, are a safety concern (i.e. it's more that the vehicle is unfit for the highway/severe grade).