I don’t see that assumption anywhere.
Where some people throw stones, there is a community with a strong tradition that teaches them “don’t live in a glass house.”
Both kinds of people travel across the ocean, and arrive in a land where many houses are made of glass. A catastrophe of disease occurs at the destination. Broken glass is everywhere. What happened? A vulnerable population was suddenly exposed to two things: stone throwers, and the secret formula for living safely in an environment that includes stone throwers.
The people who have a rule “don’t live in a glass house” are analogous to Christians. Adhering to that rule doesn’t make you guilty of yourself throwing stones.
Of course, there is merely an analogy here. People who spread disease might not even be aware that they are infected with the disease, while people who throw stones know that they are throwing stones. However, cultural practices have consequences, and the consequences might be unforeseen. AIDS in Africa is no longer newsworthy, but the problem persists. Prevention is better than cure, especially if there is no cure. Reality has no undo button.