As the expression goes "The ends sanctify the means". We generally, collectively agree that the results of the Civil War (and more universally, the Revolution) were good things, and thus the actions that advanced those ends are sacred.

Contemporary events can't be judged that way: Consequentialists (typically on the left) end up judging actions by perceived probability of success weighted outcomes, which, naturally, discount modern events since the only things with certain outcomes are those in the distant past. Deontological thinkers (more typical on the right) have to condemn contempory actions as "wrong" until they can be incorporated into a larger narrative that justifies them, which again takes time.