"It doesn't matter when it's erected and for why, it always has historical value?"

Correct.

Unlike Germany,  who prohibits anything Nazi to be remembered, the United States has tended to allow the good, the bad, and the ugly to compete in people's minds.  

It's messy--and offensive to many, but when you allow anyone to become the censor, however pure their motives, freedom is lost.  

It's like Oregon having a Board of Eugenics for 70 years to cull Oregon of defective people. They had the best of intentions, but the last Court of Appeals case in 1973 approved the involuntary sterilization of a girl who had been driven crazy by her father's sex abuse.

How many of the 2,341  “persons, male or female, who are feeble-minded, insane, epileptic, habitual criminals, moral degenerates and sexual perverts, who are, or who are likely to become, a menace to society” really should have been sterilized without their consent?

Most importantly, who was entitled to decide?  You?  Me?