I think this differs from store to store, which isn't a great policy imo. The first time I went in my SA had me try on the sizer bracelet, which is a bit different from the actual bracelet fit. I did end up trying on the actual bracelet before I purchased but my SA and I had built a relationship and she knew I was serious about purchasing ~ as soon as she put the actual bracelet on it never came off! I think they try to make sure your a serious buyer. I would assume it's also a bit of a security issue because once they put it on, it's not easily removed.
I would guess once you establish a relationship with an SA you will be able to try on the actual bracelet.