If two events are independent the occurrence of one event is not affecting the occurrence of the other event. So if event B has already occurred it will not affect the probability of occurrence of event A.
Since P(A|B) represents probability of occurrence of event A, given that event B has already occurred, P(A|B) will be equal to P(A) i.e. the probability of occurrence of event A.
This means, no matters if event B has occurred or not, it will not have any impact on event A. Event A will occur irrespective of the event B.
