Since most of the judges were from countries where the Anglo-Saxon legal system was used, that was how the trial ended up being conducted. One of the Japanese lawyers complained that "It is apparent that the procedure by which the tribunal is to conduct the trial has been drafted with Anglo-American law in mind" (Brackman 74). Due to these concerns, the Japanese government appealed to MacArthur for British and US lawyers to assist the defense. British barristers as a matter of law were forbidden from practicing in a foreign jurisdiction, but US attorneys were sent for, with one of them, Beverley Coleman, becoming the chief of the defense team. In 1981, he reflected thusly about his task:

"We were earnestly concerned not only as lawyers for justice but also for the good name of the U.S. [...] I thought that our role was the most delicate in the whole setup. If the U.S. was going to provide defense counsel, the U.S. must see to it that it be the best defense available. If we sent people to defend our enemies and they did a bum job, the good name of the U.S. in history would be marred." (Brackman 114)

A similar thing happened in other trials of Japanese war criminals. In the Philippines, a group of ten Japanese attorneys was sent to Manila in August 1947, but all but one were soon sent back after several well-publicized incidents such as that when several Japanese attacked a prosecutor. The US embassy recommended the return of the Japanese lawyers to Japan, and the defense was handed over to Philippine army lawyers. In fact, they had been assisting the defense from the beginning and helping them figure out the unfamiliar system, but now they took over the entire thing. The defendants were worried about difficulties in communication, but many came around to being content with the counsel that they were receiving. The wife of one of the defendants later commented that the Filipino lawyers "overcame their emotions and defended [the war criminals] zealously" (Chamberlain 51).

There were also US lawyers defending German war criminals at the subsequent Nuremberg trials, but only two. A motion seeking the appointment of one cited the fact that a proper defense required US counsel due to the fact that the judges and prosecutors were all from the USA. The motion was granted citing the fact that "as far as practicable, a defendant should be represented by counsel of his own choice" (Heller 164).

Basically, US lawyers were included on defense teams because they had a better understanding of the trial procedure being used. The USA put high priority on having the trials be fair and proper (in fact, they had pushed heavily for fair trials instead of something less just), and they were willing to go all the way and give the defendants US defense lawyers.

Sources:

Arnold Brackman, The Other Nuremberg

Sharon Chamberlain, A Reckoning: Philippine trials of Japanese war criminals

Kevin Heller, The Nuremberg Military Tribunals and the Origins of International Criminal Law