Because Mossberg is ignorant.
What is a "Class III" weapon then?
A weapon you need a "Class III" license to sell? Nope. No such thing as a "Class III license or permit".
Any SOT can sell them, an 01, 02 or 03 SOT can all deal in NFA weapons.
Why is the law called the National Firearms Act of 1934 or NFA? Why isn't it called the Class III law?
Marlin also calls a magazine a clip, but that is just a show of their ignorance. A magazine is not a clip and putting it as such on a box does not change anything.
If my company labeled our AOWs as "Class 42 weapon", is it a Class 42 weapon?