The issue was, if I recall correctly, that the Microsoft CDC driver was quite buggy. This led to the situation where CDC device vendors were unwilling to produce CDC compatible devices, because the default Microsoft driver would be used and, of course, the blame for any bugginess gets pinned on the hardware vendor.
Hence the current situation of many CDC-type devices that are intentionally incompatible enough with the standard, in order to require the device vendor's own drivers which they can ensure meet their quality expectations
In general, the Microchip CDC driver was free to use.
"The CDC class has been implemented in Windows (since Windows 98), macOS and most Linux distributions. Since Windows 10, no extra information has to be given in the start-up sequence as the operating system now has a generic driver that will be used for CDC. To support older versions of Windows (Windows 7 and earlier), provide an .inf file that associates with the correct driver."
However, these will not support the high-speed bit-banged JTAG mode FTDI chips can perform with the right driver. =3
Hence the current situation of many CDC-type devices that are intentionally incompatible enough with the standard, in order to require the device vendor's own drivers which they can ensure meet their quality expectations