The ban is actually at my work. It's a kennel, and the dogs are usually in playgroups for the day (monitored), and in suites at night. They've taken pitties before, usually without issues, but a month ago had an issue with one attacking and injuring a dog through a fence. No signs of an issue, no growling or fence fighting in the weeks before this, then a huge vet bill to repair the dog's ear. When there has been an issue it's like that. Other breeds usually have been fine, or if there's a fight, there's no blood or major damage.
With the current setup there's just too many risks even if that sort of dog was kept totally separated, there's no way to keep the dog totally isolated - when dogs are coming in and out they 'can' stick their faces in enough for another dog to snap, and feet/tails/small heads can go under the fences inside the building. The dogs are moved usually off leash to pens through a set of gates and isles, so they go past other dogs in the process.
When they expand they are going to make some more traditional style kennels for that sort of dog, but for now it's just not safe.