Now I can tell you have your knickers all in a twist over this. I prefer the term NFA firearm.
However "Class III" is common speak for the weapons regulated by the NFA and the people who deal in them. "Slang" I think it is called.
Do you get this worked up when someone calls a cartridge a "bullet"? The bullet is only part of the cartridge right?
Howabout calling a helicopter a "chopper"? They don't do any chopping unless the pilot is doing something very wrong.
Class III is common and accepted slang. There are several title II (NFA) dealers who refer to themselves as class III dealers on their websites. Is it because they are stupid? They are showing their ignorance? Or is it because MOST people (their paying customers) know that 'class III' means 'registered machineguns, short barrels, and suppressors'. MOST people don't know what the heck a SOT is.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.