AFAIK de-sexing contracts are pretty standard. it is very hard to enforce but understandably most breeders are concerned about dogs that they have bred with love and care falling into the hands of back-yard breeders or puppy farmers.
You are far less likely to get a great pet if it isn't eligible for registration because the health checks that most breeders perform certainly won't be done by a puppy farm or a back-yard breeder.