What did they accomplish, these depicted persons?
In the Confederate states I mean, I’m not talking about Sir Walter Raleigh (whose statues are nonetheless problematic, far more so than any Buddhist monuments)
It depends on who is being depicted, although a common view about General Lee was that he was personally against slavery and against the Secession, but that he was loyal to his home state of Virginia and he joined the Confederate cause for that reason. This seems to be one of the core ideas in the Lost Cause version of Civil War history. One who has been inculcated with that version of history would look at a statue of General Lee and not see a slaver.
Some of it also had to do with many in the North who wanted reconciliation with the South. The Northern industrialists and capitalists just wanted to do business and make money, so why should they care if the South wanted to build monuments or tell hokey stories about the Civil War? It's no skin off them.
America overall at the time seemed directed towards building a new image and identity for America which encouraged and promoted staunch patriotism and a quasi-nationalistic viewpoint of the kind which still persists today, along with strong devotion to conservative Christian values.
It's kind of bizarre to think about it, but the fact is, all of these former Confederate slave states were and still are American states, and as such, their history has interwoven into the overall tapestry of American history.
Some believe we should never have honored these people in the first place - not just the Confederate slavers, but all of them from America's past, including Washington and Jefferson. When you think about it, the whole enterprise kind of started on a shaky moral foundation. Bunch of people came over from Europe, took over the land, set themselves up a government, and rolled across the entire continent, killing and destroying entire cultures in their path.
I have somewhat mixed views about it myself, at least in terms of how far we want to take this. It's not erasing history, although it's a fair question whether we want to honor and praise historical figures even if they did things which are considered hateful, evil, and despicable. But if they did do some good things, can they be honored for that? Or does the evil outweigh the good?