The label printed on the outside of the plastic package of the chip is the only thing that's illegal, since it's depicting FTDI's trademark. Scrape that off and you've got a 100% legal compatible clone. Whoever decided to use the clones in any given device may be be in breach of a contract if the use of genuine FTDI chips was specified, but that may easily not be the case. (The clones also violate USB specs by using someone else's VID, but that only prevents them from labeling the product with the trademarked USB logo.)

Also, possession of the fake chip as an end user is no more illegal than owning fake Louis Vuitton stuff. It's the sale under false pretenses that gets you in trouble.