The difference I think you are missing is that most of these Amazon "knock-offs" are still coming from the primary manufacturer of the product. They sell them to basically anyone and will put their customer's logo on it to be resold as whatever brand they want. The mfr doesn't care what label it has and whether the person buying it knows the company logo on the side or not.

They (typically) didn't see a successful product and decide to start making a similar product to undercut the original product.