As another reply has already stated, the counterfeit chips are marked with FTDI, so the chips counterfeiters are infringing on FTDI's trademark.
But your larger question is if the counterfeit chips are (otherwise) legal. Since they're using FTDI VIDs, and misreporting themselves to the host as FTDI chips, they may be violating some standard body's rules. But other than the blatant trademark infringement, there may be no legal violation. Not that it matters, because the counterfeiting is done in China, and such behavior is not illegal there.
The IBM-PC clones pretty much did the same thing in the 80's. Except they did a proveable clean room implementation and marketed themselves as "compatible." The proveable part was important against IBM's lawyers looking for copyright infringement, and by marketing themselves as merely compatible, they were not infringing on any trademarks either. It's quite possible if these chips called themselves FTDI-compatible and not FTDI chips, they may very well be legal.