I think they feel that neutering represents responsibility. They also know that they can let you foster dogs that are not yet neutered, certainly an easier juggle. I have fostered for two small dog rescues although I have an intact dog. The applications both asked why I have an intact dog, and I wrote "on my vet's advice", which was true. BTW, they will contact your vet. I've only had one intact female, who was in heat. She was in delicate health and the foster coordinator wanted only me to foster her, trusting that I would keep them apart while at the same time bringing the poor girl into glorious good health. Long story short, she was probably not in heat, she had pyometrea (had to wait to have her spayed because vet wouldn't spay during heat/poor health). Dog's aren't in heat for ten weeks, so vet spayed her "just in time".
I'm sure there are other reasons, such as temperament issues by breed. Some dogs are very territorial when not fixed.