You mean how the driver tells the counterfeit apart? I don't really know. One possibility is that the counterfeit didn't copy some of the "don't care" cases exactly.
You completely missed the point. The driver cannot possibly know the difference between a chip falsely sold as being an FTDI chip, and one that is compatible without being mislabeled.
I got your point now. But then my comment was trying to point out that a "compatible chip" isn't really competing if its branding is deceitful. Whether it's possible for a driver to tell the difference between counterfeit and legitimate compatible chips isn't really relevant here.
Besides, any legitimate compatible chip vendor will need to have their own VID, which also means they need to develop their own compatible driver.
From FTDI's point of view, their driver's are not free, they are distributed for use by FTDI customers. FTDI doesn't want their drivers to run on non-FTDI chips regardless of whether they are marketed as real FTDI chips or not.
Counterfeit isn't the issue here so much as competing non-FTDI chips relying on FTDI to do the hard and expensive work of developing drivers for them.
Bricking non-FTDI hardware was extreme and guaranteed to make people angry. I wonder what the reaction would be if FTDI had instead made the driver not work on non-FTDI chips unless the end customer bought a license key from FTDI to use their drivers.
It'll be very interesting if they start making them as competitors now? I mean, they could.
This will not do wonders for FTDI's credibility. Indeed, it could very well put them out of business for good (from past experience), and we already know whoever's behind these clones can make a chip that does the job at a price people want…
After giving this some more thought, it's no longer clear to me that a simple {vendor, product} id pair is even "protected" by trademark. It would be protected by contract terms, especially if at some point in the supply chain someone had signed a contract with USB-IF. However it's quite clear that no entity is entitled to commit vandalism in order to enforce contract terms for a contract to which that entity is not even a party.