Why did black people at one time have to give up their seats on buses?

because of slavery laws which permitted blacks from sitting in the front of the bus

Race.

Explanation:

Back then and even in some cases now Blacks were generalized because of their race. As a result of that they had to sit in the back of the bus leading up to the middle seats. When the seats up front for the whites were taken the bus driver ordered black passengers in the middle row to stand so white passengers could sit.