Amazon must be aware of that stuff and deliberately allow it. I'd question whether it benefits them though - I've kind of been put off buying there as it's hard to know what you'll get.
It seems deliberate, as they seem to have a handle on not allowing counterfeits for big name designer goods (rolex, handbags, perfume, and so on). The counterfeits there seems to be mostly around electronics, household goods, and so on.