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.

"Illegal" is an extremely broad and imprecise way to describe a physical object.

Imagine I buy a car from a dealership, and without my knowledge the dealer has swapped out the original radio for a cheap counterfeit. I guess technically you could argue that makes the car "illegal" but it would be absurd to suggest that the manufacturer has the right to come to my house and slash my tires.

As I see it, it's not about whether "fake chip users" are obeying the laws; the burden should be on FTDI to justify its willful property damage.

To the extent that there is a violation of law involved in the manufacture, sale, or use of the chips (and such violations are much more likely in the first two cateories), there are legal remedies available for those violations, which generally require proving that a violation occurred and that the person against whom remedy is sought is the appropriate person against whom to seek a remedy.

To the extent the violations are with manufacture and sale, the user is not generally the appropriate target, even if destruction of the device was an appropriate remedy.