The "lie" keeps going because there are many people who do not know that there is no 80cc engine. So if you are an average Joe, one who never visited a MB forum, and you are searching eBay for an engine kit and you see a 67cc kit going for the same price as an "80cc" kit - which one would you buy? Some vendors do advertise "80cc" but explain that the engines are really 67cc in the description. It probably started years ago when an unscrupulous vendor decided he needed an "edge" on the competition, so he decided to add the 12cc chamber volume to the swept volume to come up with "80cc engine capacity". Then it went downhill from there.
At least we know one thing for sure, an "80cc" kit is the largest HT kit you can buy. So it's not like there is a real 75cc kit out there.