Except using your driver (and your VID/PID) is not illegal.
I'm pretty sure that this is in violation of assorted pieces of business, contract, and Intellectual property law. The license terms of the FTDI driver only allow it to be used with FTDI chips.
The FTDI driver EULA has zero legal validity, because it is distributed for free with Windows Update and there is no click-through agreement.
EULAs apply to the end user. The end user doesn't get to see the FTDI driver's EULA when Windows installs it for them automatically - in fact, I think that version of the driver bundle doesn't have an EULA attached to it at all (even invisibly), it would just be the .inf and .sys files. The EULA has absolutely no bearing whatsoever on a manufacturer of silicon that just happens to share the same interface and VID/PID. Absent an EULA that is actually visible and agreed to, the FTDI driver is only protected by copyright - and copyright says absolutely nothing about what you can use software for, only how you can distribute it. And it's Microsoft doing the distributing here.
If FTDI only distributed their driver with an installer with a click-through EULA, then they
might have a case
against users who use the driver with non-FTDI hardware (not the manufacturer! the users!). Maybe. In some countries. The legal standing of EULAs is extremely variable. But since it is distributed through Windows Update, this doesn't apply.
USB VIDs and PIDs are not intellectual property. They are not trade secrets. They have absolutely zero inherent legal protection. They are just numbers. The only legal protection they have is granted by the USB-IF's logo usage agreement, that says that you can't put the official USB logo on a piece of hardware that doesn't have a legitimately acquired and used VID/PID (roughly speaking). Therefore, as long as the clones, and products using the clones, do not use the official USB logo (and of course don't use the FTDI logo either), there is absolutely nothing legally wrong with them, and nothing legally wrong with using them with the FTDI driver supplied via Windows Update.
FTDI can kick and scream all they want, but the law doesn't guarantee anyone a market, a monopoly, or exclusivity and control over your products. There is copyright protection, there is trademark protection, there are patents (not discussed here), and there is contract law. Without a contract (EULA), with no trademark infringement (FTDI and USB logos), and with no copying done by anyone other than Microsoft, they have nothing else to stand on.