emperor's new horse.
Let's say that I see the horse and I do realize that it's an assistance animal, albeit unusual. If other people doesn't look so surprised I might suspect my ignorance: it seems that all other people know what's going on, and take it as perfectly normal. As I don't want to stand out in the crowd as the only freak who didn't know about this socially accepted habit, I simply play quiet as nothing strange happened.
The same thing could happen in the head of many people and the overall effect would be that of uninterested people not caring about anything.
Group psychology is a very strange and fascinating thing.
(The analogy is with the tale of the http://en.wikipedia.org/wiki/The_Emperors_New_Clothes)
A logical explanation doesn't change the analogy. I'm aware that S&M Gimps exist. If I went into an Apple Store where someone was being led around by a gimp, and no-one was paying attention, I'd still think that was unusual.
Not to mention that an aside explaining that helper horses exists detracts from the art of storytelling.