Because ICTY sham was created by the US in clear contravention of UN Charter. It has nothing to do with the international law.
ICTY was created in haste and there is ample evidence of it's illegallity. UN Charter is the top UN document and it describes the powers of each UN body. UN Security Council (UNSEC) does not have legal powers and therefore can not create an auxilliary body with legal powers. All legal power of the UN resides within the World Court, and General Assembly. On top of that, the reason for violation of the UN Charter was to punish alleged Serbian aggression on Bosnia, i.e. the motive for creation was political. That alone puts this sham into the same drawer with Hitler's and Stalin's "courts" and political trials.
The "justice" dispensed in ICTY is neither Continental nor precedent law. It is very likely that any American lawyer having a stint in that circus could be disbarred in his/her local jurisdiction upon returning home. That's why many who served there keep mum of their "success story".
OK. I agree that the legality of the ICT is questionable.
But the US official was not employed or involved with this court. So this guy relying on assurances from Holbrook is a lot like a guy talking with a cop in Maine about a bargain and claiming that he is therefore protected against prosecution by the state of Alaska.
Holbrook never had any authority to strike deals for the ICT.