The reverse-engineered reimplementations / clones would have to have been programmed with an FTDI VID (or included modified FTDI drivers) in order for the FTDI drivers to claim them, so I'm not so sure about the "not marked as being from FTDI" bit.
That's an interesting question. Is the VID a signal of protocol compatibility? Or a brand name?
In a way it's both, but since end users almost never see the VID, I suspect a lawyer would have an easy time that under the law it's more the former than the latter, and therefore not covered under trademark and related laws.
Especially since it's the only way to be compatible as far as I know (since (VID,PID) determines the driver that will get loaded for proprietary protocols).
Also.. Can you even claim any special rights to a non-government registered trademark/brand name (one that is registered at a non-government entity - the USB Implementers Forum in this case)?
It seems like trademark would be really difficult to attach to (VID, PID) if no one in the supply chain actually used any USB-IF logos. Basically FTDI assumed a right to destroy property based on a violation of a contract neither the property owner, nor any of the chain of vendors that produced the property, had been a party to.
