If the Amazons existed, and there is some debate about it based on ancient texts, I can't see why they would have removed one or both breasts, there doesn't seem any logic at all to it.
Modern female archers don't need to do this to achieve accuracy, and even if their breasts got in the way, binding them would have worked as well for Amazon hunters, who would also have been conscious of the need to build the tribe's wealth and strength through creation and feeding of children.
It's more likely that as part of a warrior culture, women warriors in ancient times would have undergone the usual rituals of scarrification, mutilation, tattooing and etc with which tribes around the world tested those who would become members of the warrior caste.
'Reporters' from other cultures (such as Greek scribes who travelled with armies to report their glorious conquests) had a vested interest in making the enemy seem as fearsome as possible and the home team in a favourable light. And, let's face it, they were also the ancestors of today's tabloid writers!
Stories from the front lines about hordes of grand and savage warrior women who lopped off their breasts in order to fight is a MUCH better story than a hundred desperate and starving flatlands tribal warriors who comprised male and female, as many did. Some also used child warriors, much as we read about child soldiers in some places today.