I have read a series of horror stories in another forum - about SBT killing other dogs. Not that they should have been allowed to exercise with other dogs - especially very small ones - but once the fight is on - even if the exercise yard is well supervised - it's hard to stop another dog from being seriously hurt.
But I wonder if the particular kennel has a no Husky, GSD, Malamute etc policy too.
One of the places I board my dog - has limited exercise area - so they check how each dog gets along with the other dogs before allowing them to run together. It would be really inconvenient but not impossible for them if they had to run some dogs on their own.
So I guess - if the kennel has had previous bad experience with SBT or other breeds - that they might ban them. But that means it's the wrong kennel for SBT (and probably any other dog) anyway.