Even when employment is "at will", permanent employees of large companies are generally protected from abrupt job termination by severance policies, like advance notice in case of layoffs, or formal discipline procedures.
Permanent employment is often considered the gold standard for employee life quality and society development as it gives stability and peace of mind to citizens.
It has been seen that permanent employment gives better results for social development in the long term because of various reasons such as steady circulation of liquid wealth through wages, peaceful society, regular upskilling by companies etc.
[1] Japan's lifetime employment originated in large companies around 1910 but became widespread during the economic growth period following World War II.
In the enforcement of occupation policies GHQ dissolved Zaibatsu and emerging labor unions had started to claim for a higher standard of benefit.