Because some dogs will absolutely attack unprovoked.
A few weeks ago, I did a 5K which went through the woods. My husband and son (almost 2) came to support me. There was another runner there who had a dog. We were hanging around before the race in the vicinity of the runner with the dog. My son loves dogs and he smiled at the dog. He didn't walk toward the dog or reach out or anything. Just smiled. The dog lunged forward and started barking. Fortunately, the dog was on a leash and the owner was paying attention, so no harm done. I guess if you want to consider smiling at a dog "provoking" it, then he provoked the dog. But at a place like WDW, many toddlers are going to smile and maybe even point or wave when they see a dog. If that sets the dog off, there could be problems, particularly if their owners are busy with check-in and the like and maybe don't have as good of a hold on the dog as they should. It really only takes one bad incident to set off a slew of negative publicity. Look at the flak they got for the alligator incident, and that was a wild animal (not one expressly welcomed on property by Disney).
Another instance - my MIL used to have a poorly behaved chihuahua/beagle mix. That dog would scratch me nearly every single time I visited and attempted to bite me on more than one occasion for no apparent reason. I suppose I could have gotten a minor injury (and she did draw blood scratching me once), but that dog wasn't big enough to cause great harm. Still, it's an example of another dog that would attack without being provoked.
I wouldn't worry about a service animal doing such a thing, because as I understand, they receive very specific and strict training not to interact with humans aside from their handler except on command.
