If the plane is in the US, the crew should be subject to the same post crash checks as a US based crew.
As NTSB mentioned, U.S.-registered airlines operate under Part 121 of the FAA regs, while foreign airlines operating into the U.S. operate under Part 129 of the FAA regs. Most simply stated, Part 129 kicks things back to regs of the foreign airline's country, which may not (and often don't) mirror the FAA's regs. Under Korean regs, the crew may not have been required to submit for post-accident testing.