I think it was a heroic act... But if it had happened a few months earlier- before tensions had reached a sufficient level- we most likely would never have heard of her... It would have still have been a heroic act, but just another act of personal heroism that occur countless times every day.

At that time, on that bus, in that city she had the courage and dignity to refuse to be treated as an inferior and to stand by her beliefs. She is remembered because 1) she was a hero, 2) because she was morally right, 3) It happened at a time where outrage was at a point that her act would be the spark to ignite a movement.