Beyond needing us as a trading partner Japan needs the US for protection against China paying them back for the 1st half of the 20th century and lacking a strong military and above all, nuclear armaments they need to stay on good terms. So while they will still look to their own interests and engage in protectionism for their own industries where they can pull it off, they are less likely to act unfairly with impunity towards American companies.
Thus Japan is safer for a company with cutting-edge tech to do business in than China.
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