In the case of our shelter here...I am all for adopting out unaltered cats. I have broached the subject a few times...and I have gotten the same answer. They simply cannot afford it. We have never seen an animal come back with offspring, or gotten kittens/puppies from an animal we adopted out, but I think they keep a close watch on the spay/neuter certs & make sure animals are altered. Actually, right now, the shelter is to the point (financially) that they are deciding if it is worth treating ringworm in some of the cats, or if they should be euthanized.
There simply isn't enough funding, and WE MUST put up an addition soon, or we will have to euthanize for overpopulation again(we haven't had to for 4 years until a few weeks ago).
I do think that it is so important to have animals spayed/neutered before adoption, but if the shelter here did, they wouldn't be open for much longer. The city only gives minimal grants & the public just doesn't donate enough. They don't know the difference between the building addition fund & animal care cost donations. If we feed a kitten for one day, fontline, deworm, FeLV test, & give the first vaccination, the $50 adoption fee is spent. The fee was higher, but no one was willing to pay it for "just a cat".
I spoke with all the vets in the area about pediatric spays/neuters, one said he didn't see a need to fix so young(he & I don't get along), one said she is uncomfortable with the idea(she specializes in large animals), and one said he didn't like to be operating on such small babies. They all know the benefits of it, too.
IMO, if the choice is have a shelter, of fix a few pets prior to adopting & then have the shelter close....I think pets should be adopted out unaltered. Keep in mind this is a shelter as well as the city pound, so we MUST take in all strays & have room for them. Supposedly, we are only supposed to house 35 adult cats, but typically we have 10 or more over that limit.