This is interesting , in the INF file for the latest FTDI driver 2.12.00 this text has appeared
; Copyright © 2000-2014 Future Technology Devices International Limited
; USB serial port driver installation file for Windows 2000, XP, Server 2003, Vista, Server 2008,
; Windows 7, Server 2008 R2, Windows 8, Windows 8.1 and Server 2012 R2.
; IMPORTANT NOTICE: PLEASE READ CAREFULLY BEFORE INSTALLING THE RELEVANT
; SOFTWARE: This licence agreement (Licence) is a legal agreement between you (Licensee or
; you) and Future Technology Devices International Limited of 2 Seaward Place, Centurion Business
; Park, Glasgow G41 1HH, Scotland (UK Company Number SC136640) (Licensor or we) for use of
; driver software provided by the Licensor(Software).
; BY INSTALLING OR USING THIS SOFTWARE YOU AGREE TO THE TERMS OF THIS LICENCE
; WHICH WILL BIND YOU. IF YOU DO NOT AGREE TO THE TERMS OF THIS LICENCE, WE ARE
; UNWILLING TO LICENSE THE SOFTWARE TO YOU AND YOU MUST DISCONTINUE
; INSTALLATION OF THE SOFTWARE NOW.
; 1. GRANT AND SCOPE OF LICENCE
; 1.1 In consideration of you agreeing to abide by the terms of this Licence, the Licensor hereby
; grants to you a non-exclusive, non-transferable, royalty free licence to use the Software on
; the terms of this Licence.
; 1.2 In this Licence a "Genuine FTDI Component" means an item of hardware that was
; manufactured for, and sold by, the Licensor or a member of the Licensor's group of
; companies. It does not include any counterfeit or fake products.
; 1.3 If you are a manufacturer of a device that includes a Genuine FTDI Component (each a
; "Device") then you may install the Software onto that device. If you are a seller or distributor
; of a Device then you may distribute the Software with the Device. If you are a user of a
; Device then you may install the Software on the Device, or onto a computer system in order
; to use the Device.
; 1.4 In each of those cases you may:
; 1.4.1 install and use the Software for your purposes only; and
; 1.4.2 only use the Software in conjunction with products based on and/or incorporating a
; Genuine FTDI Component.
; 1.5 The Software will not function properly on or with a component that is not a Genuine FTDI
; Component. Use of the Software as a driver for, or installation of the Software onto, a
; component that is not a Genuine FTDI Component, including without limitation counterfeit
; components, MAY IRRETRIEVABLY DAMAGE THAT COMPONENT. It is the Licensee's
; responsibility to make sure that all chips it installs the Software on, or uses the Software as a
; driver for, are Genuine FTDI Components. If in doubt then contact the Licensor.
You have all read and agreed to that haven't you? :)
So it seems that FTDI are now deliberately disabling counterfeit devices.
The license stated in the INF file is not legally binding, since the driver is installed automatically without the user seeing and given the opportunity to decline the agreement [IANAL]. Disabling user's property is probably illegal, but I doubt you'd win any suit in court.
FTDI are perfectly within their rights to only support genuine products, but I am not sure disabling clones is a sensible tactic, particularly when there is a trivially easy way to circumvent it. They are probably looking forward to the day when only Microsoft approved drivers can be installed on Windows, and it is a crime to tamper with your Windows installation!
I didn't see anything in the INF file that disables counterfeits, so either the change happened in an earlier release or more likely the code is in the binary drivers.
The advice to "only buy from FTDI authorised sellers" may as well be "don't buy FTDI", since not even big suppliers like RS and Farnell are authorised sellers. You'll be ok with Mouser and Digikey though :)