The Japanese started this whole thing during the Sino-Japanese War in 1937. The first confirmed case was over Nanking on Sept. 19, 1937 when the pilot of a Boeing 281 (export version of the P-26) was shot and killed in his parachute by JNAF aircraft. Pilots from both the JNAF and JAAF did this routinely. Later in the war, Chinese and Russian volunteer pilots often delayed opening their parachutes to avoid being strafed. Even after a safe parachute descent, the Japanese could still go after you. In July, 1938, one Russian volunteer, Valentin Dudonov, bailed out and landed on a sand bank in Lake Poyang after a collision with a JNAF A5M. Another A5M came and strafed him on the sand bank. Dudonov had to jump and hide under water in the lake to escape!
The Chinese certainly refrained from the practice. They had learned very early on that the Japanese aircrew was MUCH more valuable alive.
One of the first two Japanese aircrew captured on the second day of the fighting in Shanghai (Aug. 15th, 1937) defected to the Chinese side. This observer from the Kaga Kanko (torpedo bomber) unit was very valuable in helping decrypt JNAF radio traffic.
The Japanese code of conduct ("bushido": Way of the Warrior) considers being captured a great dishonor. In many Japanese accounts, there is a standard phrase used to describe the person was captured only because he was rendered unconscious (to preserve honor).
Since the dishonor extends to the family of the captured, PoW often preferred not to let their families know and suffer disgrace. During the Japanese civil wars, combattants followed this code of conduct. But once captured, the combattant would have lost everything and, essentially, can not return to his own side. It is not unusual for the prisoner to switch his allegiance to his captors.
The Chinese Air Force took advantage of this and "turned" two JNAF aircrew (the Kaga observer described earlier and a 13th Ku fighter group leader captured a month later) to join their side. Even before Herbert Yardley went to China, the Chinese already had some success breaking Japanese tactical codes. One of the most successful teams was led by a mathematics professor who was trained in M.I.T. Consequently, every effort was made to capture Japanese aircrew (alive) for intelligence purposes.
It is true that many JNAF and JAAF aircrew chose not to bail out when they could have done so to save their lives. But when they do, it made much more sense to capture Japanese aircrew than to retaliate for Chinese aircrew being shot at in their parachutes.
Sometimes, however, things did not work out so well. Hsu, Chi-hsiang, who later became a squadron leader in the Chinese-American Combined Wing (CACW), recalled an incident of Chinese ground troops shooting a JNAF pilot descending in his parachute. Late in 1939, the Chinese 4th Pursuit Group deployed their I-15bis to Liuchow, Kwangsi to support the Chinese counter offensive to re-take the Kunlun Pass. On Dec. 30th, 1939, thirteen A5M's from the JNAF 14th Ku caught the Chinese while they were coming in to land at Liuchow. A number of I-15bis, including one flown by future ace Lt Chow, Chi-kai, suffered damage. Although the Japanese claimed 14(!) kills, none of the Chinese fighters were actually lost. Hsu recalled that the attacked came so suddenly that most of the Chinese pilots did not even have time to evade. The exception was a junior wingman in Hsu's 23rd Pursuit Squadron Lt Liu, Kai. Li managed to pull behind the A5M of PO2c Fujita, Hiroshi and get in a telling burst using his 4 ShKAS machine guns.
Fujita bailed out of his stricken aircraft and landed near the airfield. After landing in Liuchow, the Chinese pilots crowded around Liu to congratulate him. They then went to the airfield security unit to see if the Japanese pilot was captured. There was even talk about inviting the Japanese pilot to join their New Year celebration. (Quite a few of the Chinese pilots were rookies and had not seen much action up to this point).
Unfortunately, word came back from the airfield security unit that Fujita was dead. One of their men shot the Japanese pilot while he was descending in his parachute ...
Later in the war (1944-45), when the Chinese and Americans were taking the war to the Japanese airfields far inside Japanese held territory, there were recorded instances of Chinese and Amercian pilots gunning down Japanese aircrew in their parachutes.
Much had happened in the intervening years to embitter those young Chinese pilots who were planning to invite their captured foe to a party. After the JNAF introduced the Zero-sen in September, 1940, many witnessed comrades killed, including quite a number strafed in their parachutes. Hsu himself survived a crash landing in a I-15bis during the combat debut of the Zero-sen Sept. 13, 1940. Others saw or heard accounts of atrocities committed on Chinese civilians ...
After all of that, it is little wonder that Japanese aircrew in parachutes were considered "fair game" ...