So if someone is not familiar with your social rituals then they are not to be trusted?
Yes. This is the basis of pretty much all Western human interaction, from the observations and data I have collected over the last 30+ years. It is the root of all inter-group conflicts in the country, from the lofty halls of politics to the "that group's not really a metal band!" subreddit pettiness.
Humans are ritualistic and their interactions are so rigid as to be almost mechanistic, when you get down to the base of them. Every person isn't so much a unique individual as they are a unique combination of common parts, and their communication ceremonies reflect that.
Because someone who doesn't want to shake hands because it is taboo in their culture is the same thing as someone refusing to check the flaps before takeoff.
Yes. That is exactly correct. If you don't do the ritual right (or right enough, within a margin of specification), you will not be trusted.
Does it make rational sense from the perspective of a sapient being capable of examining their own actions? Fuck no. But that's the world we live in. We refuse to learn it and adapt to it at our peril.