Yeah, it's the trauma. That's the point. The potential cost of a single violent interaction is extremely high. There is relatively no reward for being pleasant

Yeah, it's the trauma. That's the point. The potential cost of a single violent interaction is extremely high. There is relatively no reward for being pleasant to the 99% who just happen to look an awful lot like the one who tried to stab you. It is the same rationale behind profiling. Which is to say, it is rational, just ineffective and with a number of bad side effects.