Edit: I just realized I responded from the adoption standpoint. I didn't address fosters. I guess for fosters it's that the foster parent was the one who ultimately made the decision about where the dog should go, so if they were pro-unaltered dogs, they'd probably be willing to adopt the dog to an unaltered home, which was a no-no for reasons listed below.
I am curious about this logic. What are the reasons generally given for rejecting a foster home with intact animals? From a rescue standpoint, do they think you're unable to handle keeping dogs separated (like when a female is in heat)? Or do they think you must be looking for free breeding stock? Or... Is it like, wanting you to be on board with a particular mentality? (Alter everything.) Do many fosters go to their foster homes before being altered?
The rescue I used to foster for didn't allow dogs to go to homes with unaltered animals. I *think* (major assumptions here, I didn't make policies or really talk to the higher ups) it was two fold. One, we often did have unaltered foster dogs. They came straight from rural shelters, and were generally here for a while before they were fixed (especially puppies). Second, the rescue had a "reputation" to uphold, for lack of a better term. People LOVE to talk/gossip about rescues here. If word got out that the group was condoning backyard breeders by allowing them to adopt animals, the rescue would have been blacklisted and fallen apart in seconds (it happened to one locally a few years ago).
That rescue had no shortage of adopters. They adopted over 1000 dogs in their first 18 months, with full vet checks and home checks and behavioral analysis on every dog that came through. It was easier to make a blanket policy that said no unaltered animals (save for medical reasons that could be confirmed by calling the vet) than to dig through and figure out how responsible this responsible person really was, and risk the reputation of the rescue.
For better or worse, it worked for this group. They weren't one who held onto dogs for 9 months waiting for the "perfect" home that never materialized. There were plenty of homes, and the average dog only stayed with the group for 3-4 weeks.