'Compatible' chips that report FTDI's USB Vendor ID (VID) and Product ID (PID). That way, they don't have to actually write their own driver and get it approved by MS.
So, when Windows interrogates the device, it appears to be FTDI, so Windows loads the FTDI driver.
That driver makes an undocumented call that only genuine FTDI chips will respond to correctly, so the driver can tell whether a knockoff part is attached.
Other legit serial chip makers use their own PID/VID, so it's not an issue with TI, Silabs, etc., only with 'Best Lucky Interface Ltd' parts.