How do cats with Feline Panleukopenia get into cat rescue facilities?

Catawba animal shelter I believe
When a cat in a rescue center falls ill with feline panleukopenia it is a catastrophe for the other cats in the same facility because they might have come into contact with the cat who is very ill and because feline panleukopenia is highly contagious and often a fatal disease, veterinarians decide for the sake of safety to euthanise all the cats that may have come into contact with the sick cat.
This has happened in a shelter in Catawba, which I believe is in the state of North Carolina but somebody who knows better than me might wish to correct me.
Last weekend, shelter staff members found two cats at the county’s animal shelter who were dead in their cages. Tests concluded that the cats died of feline panleukopenia (feline distemper).
At that moment in time, and after, I suspect, a lot of thought and with a heavy heart, shelter staff decided to euthanize 87 cats who might have come into contact with either one of the two dead cats.
The shelter staff consulted with their veterinarian and weighed the potential for the virus to spread to other parts of the facility and into the community against the seriousness of extensive euthanasia, and decided to euthanize to minimize the risk of this nasty virus spreading.
This has happened before at another shelters and it will happen again but why should it happen? Does it have to happen?
I don’t think we can criticize the veterinarian and the shelter staff for making their decision but I believe that we can criticize somebody at the shelter facility for taking into the facility two cats who carried panleuk or perhaps they got the disease from cats inside the shelter? It doesn’t change things.
If the consequences of introducing a sick cat to a shelter are this catastrophic surely it makes sense to have a quarantine area where new intakes can be held pending with certainty that they do no carry serious diseases.
There have been other outbreaks and the same sort of mass euthanasia in response, in other North Carolina shelters.
The root purpose of cat shelters is to save lives through adopting cats out to new caretakers.
To euthanise 87 cats, many of whom were no doubt excellent pets and healthy, is a complete failure of management.
Perhaps I am oversimplifying things. Perhaps I missed something important. I am not an USA cat shelter expert. But when foster carers of several cats pull a new cat from a shelter they have to make sure, at the outset, that the cat is not carrying a nasty contagious disease because they risk the lives of all the cats in their care.
Also adopters from shelters have to be sure that the cat they adopt is healthy. There seems to have been an oversight in this county shelter. How commonplace is this sort of risk?