An oft-cited passage from the Gospel of Matthew (6:24) goes: "No man can serve two masters. For either he will hate the one and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon."
It's easy to see why. Serving two masters can easily create a conflict of interest, particularly in the case of staff with knowledge of the inner workings of their employer.
Not so long ago Japan's lifetime employment and other paternalistic practices demanded a worker's complete loyalty, and moonlighting (referred to in Japanese as fukugyō or, literally, supplemental or secondary work) was discouraged or banned outright, a situation that remains in force for government workers and certain parts of the private sector.