I have read many post on many subjects over the years. When a statement is made that something is or is not legal, I typically ask for a reference to that particular law. The reason I ask, is to avoid repeating misinformation. A good example is the venders that publish the towing laws by state. Many are incorrect, but do help them sell their product.
I do not know if there are any laws on hitches or extensions, but I have never seen one identified in a vehicle code. I have never heard it is illegal to make your own hitch extension. As for stickers stating the legality of a given product, I would like to see the vehicle code that supports the claim. It is very common, and I think a good thing, for manufactures to suggest information about the use of their products. If something is the law, cite where is in the vehicle code.
I have no axe to grind with any hitch manufacture, but sure would like truth in advertising. My post on this subject is not intended to reflect negatively on anyone. I will continue to ask for verification if something is said to be the law. I think it is too important to be ignored.
Wayne