Why are these people at the Macklin Ave Humanes Society keeping a dog that was rescued and on its way to the vet. They want to put it down.

The Humane Society of Missouri no longer puts down animals. If the animal does not get adopted, then they have a forever home there at the Humane Society. If the animal is abusive to any person who takes care of it, or there is some other way the animal can not be taken care of, then the choice to put the animal down will be considered.

Legalities is my best guess due to medical issues, unknown to you. Don't let this deter you in your overall mission.