Because treason is treason. There is no historical evidence that we have treated those who have committed the worst crimes against mankind differently because it was in the past. Hitler is not revered because it was over 70 years ago. No one is ever going to erect statues to revere Ted Bundy or Charles Manson. No one is going to put up plaques to revere slave traders or the Roman soldiers who crucified Jesus. There are things that are universally wrong and treason against your country is one of them.
Interesting. No statues either in Spain or the conquered Americas of Cortes or Pizzaro? No public tributes, statues or monuments to Ferdinand and Isabella? No cathedrals from the 15th century left standing?...a church that though they may not have crucified Jesus - crucified much that he tried to teach us.
My hope would still be though, if those statues and monuments exist - they are not torn down in a misguided, or nefarious, attempt to erase history. Destroying works of art in the public square is very ISIS worthy.
There may be no statues of Hitler on US soil - but you can find Che and Lenin. One killed millions, the other no less a brutal murderer, just a smaller sphere of influence. They stand only because they are heroes of the Left - and the Left are the self-appointed censors of our time.
You are aware of the significance of presidential pardons. Yes?
http://www.presidency.ucsb.edu/ws/?pid=72360