The US officials you gave as example are simply retarded and/or set up and/or part of a PR campaign.
Sorry, "here" is Brazil.
They were not particularly retarded, just did not imagine that they were under police surveillance. The bribes were in cash because it was the only viable alternative; bank transfers would obviously not do. (In another case the payment medium was an expensive piece of jewelry. But the bribee was caught when he tried to sell it at a jewelry store, because, by bad luck, that piece had been stolen from that same jewelry store a couple of months earlier.)
You are very deluded if you believe bribe money passes through the personal bank accounts of those involved.
Most bribes of that magnitude (a few tens of thousands of dollars) probably do. Money starts out in personal bank accounts and must end in personal bank accounts to be conveniently used. Withdrawing or paying with big piles of cash calls unwanted attention.
Amounts involved in serious corruption or drug traffic (hundreds of millions) are too big to use BTC, and do not need it. Did I mention that a couple of months ago our federal police caught a helicopter shipment of 450 kg of cocaine (one of a regular series it seems) in a farm here? The helicopter belonged to a Senator, the pilot worked for the Senator, the farm belonged to the Senator, but to this day the police does not have a clue as to who could possibly be the owner of that cocaine.
Finally, if someone were to use BTC for bribe money, be certain the transfer would be thoroughly sterilized through a bunch of exchanges and other BTC hygiene services worldwide. Do you seriously believe there is any way to stop or detect this? For all I know there already exists a BTC laundromat for oligarchs.
I don't think that opening accounts on exchanges outside China would be easy for a typical Chinese small businessman. Doing that sort of thing would require familiarity with the bitcoin technology and markets, not likely for such people.
If I were the FBI or NSA, I would set up dozens of bitcoin tumblers out there, with the cheapest rates and best service. It would save a lot of time compared to tracing coins through the blockchain.
For all I know there is a lot of wishful thinking about "chinese oligarchs adopting BTC"...