I understand completely why they would do this, even in the absence of solid scientific evidence. Put on your lawyer hats for a moment and think of it this way: If an immune compromised patient gets an infection and dies, and the family of this patient gets it in their heads that the only way the patient could have obtained this infection is from the therapy dog who visited a day or so ago, they're going to get on the phone and contact a personal injury lawyer (who will work on a contingent fee basis, meaning the more his/her client is awarded, the more he/she makes off the case). These lawyers are bulldogs (no offense meant to actual bulldogs, btw) and they don't care if your dog is healthier because it eats raw meat; all they know is that raw meat has bacteria, their client's loved one succumbed to a bacterial infection, ergo, the dog was the proximate cause. They only have to convince a jury that any reasonable person would draw the same conclusion. Most "reasonable" people have not researched raw canine diets to the extent that some of you have.
Because the Delta Society is an organization that carries liability insurance on their Pet Partners, they might be viewed as a "deep pockets" target for a personal injury lawyer. Not that they are--it's actually the insurance carrier who is viewed as having lots of money.
And, if we imagine that this hypothetical deceased patient was under the age of 20, and did not have a terminal illness, the potential award could be in the millions. So, the policy may not be coming directly from the Delta Society, but from their insurance carrier. Litigating these types of cases can cost more than the settlement itself, and a verdict for the plaintiff could prove to set a dangerous precedent for future cases. It could ultimately have consequences for all therapy dogs, not just the Delta ones, especially since the facilities the dogs are visiting may be at risk of a lawsuit as well. This is why it is their business what therapy dog owners are feeding their animals.