They have no call to act like that. You are doing them a great service by helping, but, as I have seen they think you should know this might happen and you should deal with it. But if your dog wag injured do you think for one minute that they would pay for the vet bills....NO. I use to be an ACO and have done fostering for the shelter we worked out of and I had to pay for any vet bills.
Maybe you could find another organazation to help who would understand that our pets come first.