I would think it wouldn't be worth the trouble to make knock off speakers, and I am guessing that is why you rarely see them. Speakers aren't like handbags or shoes. Their construction is more involved, and its much harder to duplicate cabinets and woofers of hifi speakers as opposed to the stuff that's normally knocked off. It simply is not a cost effective item to knock off. It's also a smaller and more tightly-knit community. If someone is selling knock off speakers, word would spread quick and that seller would be shut down fast.
There are
'white van' speaker lines of cheapo imitator brands, but they are not knock offs. Crap speakers disguised as hifi speakers are not illegal in themselves, although perhaps a bit unethical.