If they didn't need transportation, for example if they drive, that isn't an issue. Some won't need it for the line itself. There were enough around to know there are exceptions being made. One even had a sticker or something on it saying it has been checked by Disney and is indeed allowed in the parks, but not on the buses and it was clearly not a fake it if it was, it was the most impressive one I have ever seen. And you are right, they wouldn't fit on attractions, but again this using them might not need it for that, it might just be for distances.
I am not saying that some shoulder or shouldn't be allowed, just that they are. I would estimate having seen about 15 over the 10 days that were distinctly unique. But trying to be difficult, just saying a one of, I would agree, but there were enough that it wasn't a one of, but not enough to say it was super common.