If he walks, without imparting his knowledge is he selfish? If he tells only a few people is he cowardly? If he tells everyone is he spoiling everyone’s trip?
I suspect, if we were related to a victim, we would expect him to take courage and shout to all. However if the same ethics are applied to someone who knows that the food being served will cause a health crash, he is often coerced to remain silent and join the doomed flight to death.
The scientific findings are perfectly clear; eating meat and milk invokes suffering and death from chronic diseases.
Many of us do not wish for anyone to unwittingly suffer, or die, so we are inclined to shout the warning.