It makes sense when dealing with that many people. It’s a simple numbers game at that point.

Let’s say we have a massive evacuation that requires large amounts of people to live on top of each other. Those people need things such as food, supplies , medical care etc etc.

Now if you let a known virus into a situation like this is causes multiple problems. These problems can and will put a threat on the system. This causes the supplies and medical care to take a hit.

All this can be avoided if the population you save is free of a disease.

It’s simple math. Humanity is thrown out the window when we are so vast in numbers we are reduced to nothing.