One of the top goals of sales is to get the sale closed as quickly and efficiently as possible. In that regard, this is a failure on Apple's part.
If the store is near an area that has a large deaf population, I can see where it would make sense to have a salesperson that can cater to those people, and at times when there are no deaf customers it would also make sense for that employee to try to help other customers. However, if this is just a case of a random deaf person wanting to work in retail, or at an Apple store, it would seem like there would be roles potentially better suited for the person. Few, if any, non-deaf customers are going to really enjoy that kind of interaction in the sales process, and I could easily see it causing some customers to just give up and buy something else (maybe still an Apple product, but at a different retailer, or maybe a non-Apple product).
Another angle could be if this person was super knowledgable about the product, or had some other way that made his ability to help customers offset the friction from the interactions, but from your description that did not seem like the case.