Actually, it is not. "Their" USB VID/PID can legally be used by anybody, it just means that the USB logo may not be used. AFAIK (and just checked on some FT232 I have), there is no USB logo on these chips.
Oh really? Not according to this FAQ:
Because a USB VID is specific to a particular organization, derivatives shouldn’t use the VID and PID of the original hardware.
Regardless of the fact that it may be legal for others to do so, it's unethical and clearly misrepresentation. It's like when Palm tried to use the USB VID of Apple so iTunes would think the Palm Pre was an iPhone - great for Pre users until that causes crashes or data corruption for users and Apple could be held liable.
Rightly so, Palm was slapped down for their "reuse" of Apple's VID.