@dashen,
@cyro11,
@the5thseraph
Seraph's right, that's basically a boob protector, not a heart shield. It's for when she uses her bow, in case the string scrapes her when she fires. In some tales of the Amazons, they would do certain things to their warriors to either prevent that breast from growing, or just remove it, all for the sake of better archery.
In practice, she should be able to find a way to hold her bow so the string doesn't hit her. In the field she may not be in a place where she can hold her bow normally, and would rather risk a strike than lose her target. Thus the boob shield.
@The5thSeraph You've heard busty women complain about their bras, right? Besides, this is not about support. It's about protection. Also, if she's holding her bow in such a way it could strike both breasts, the shot would go way off target. She would learn quickly to not take that shot.