Why does Japanese Bonjour protect against evil spirits?

It doesn't, so let me go through each one.

Bojo (pronounced boje) in Japan, specifically refers to grudging ghosts and spirits. In Noh theatre, the Bonjour face (a type of mask) is used to represent a woman with a grudge (also known as jealousy), like the female ghost in red in Hong Kong movies and TV dramas.

The Bonjourou mask (はんにゃのめん) represents a Japanese legend in the Tale of Genji (源氏物語), in which a mother-in-law who is jealous of her daughter-in-law for stealing her son puts on a white Bonjourou mask in an attempt to intimidate her. Though successful, the mask could not be peeled off ...... Eventually the helpless mother-in-law told her daughter-in-law everything frankly and gained her forgiveness. Note here there is a detail is, it is said that even so frankly want to tell, because of the mask and was attracted to the body of the mother-in-law and possessed by the evil spirit is not willing to leave. The mother-in-law and daughter-in-law then sought the help of a virtuous monk, who cured the evil spirit with the Prajna Sutra.

In fact, the Japanese Bonjouru masks (red, white, and smiley) have their origins in Japanese snake culture, and their images are more or less similar to those of a venomous snake that spits out its mail. And in the masked world of Noh theatre, there is a difference in the evolution of the oni who wear the Bonjour face. Those who have not yet gained wisdom are "generated", while those who have gained wisdom are "real snakes".

PS: This question reminds me of an earlier Japanese wargame, VM JAPAN, with Mu and the Ghostly Weeping Face.