The intent is to stop glorifying their legacy by having a statue of them up. This does not equate to erasing them from history as this will still be taught.
But it's not glorifying their legacy - it's simply an acknowledgement of the stature they had in their own era.
It's not glorifying them
now. It's simply a monument to their time and how they were perceived
then.
It's like this:
The point of something like this isn't to glorify "war" itself, it's to simply acknowledge it happened. Tearing it down because "wars are bad" would be monumentally stupid.
Statues are basically mini-museums. It allows people to look at them, see a name plate and then if they want do independent research and learn from why it's there. It does nothing else.
For Edward Colston, it'd lead a lot of people here -
https://en.wikipedia.org/wiki/Edward_Colston - where it says clearly he exploited slaves.